Move /utilities/flexbox to /utilities/flex to match name of SCSS file

docs/4.0/components/forms.md

@@ -534,7 +534,7 @@ And of course [custom form controls](#custom-forms) are supported.
 
 Use the `.form-inline` class to display a series of labels, form controls, and buttons on a single horizontal row. Form controls within inline forms vary slightly from their default states.
 
-- Controls are `display: flex`, collapsing any HTML white space and allowing you to provide alignment control with [spacing]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/spacing/) and [flexbox]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/flexbox/) utilities.
+- Controls are `display: flex`, collapsing any HTML white space and allowing you to provide alignment control with [spacing]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/spacing/) and [flexbox]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/flex/) utilities.
 - Controls and input groups receive `width: auto` to override the Bootstrap default `width: 100%`.
 - Controls **only appear inline in viewports that are at least 576px wide** to account for narrow viewports on mobile devices.
 

docs/4.0/components/list-group.md

@@ -109,7 +109,7 @@ Contextual classes also work with `.list-group-item-action`. Note the addition o
 
 ## With badges
 
-Add badges to any list group item to show unread counts, activity, and more with the help of some [utilities]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/flexbox/).
+Add badges to any list group item to show unread counts, activity, and more with the help of some [utilities]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/flex/).
 
 {% example html %}
 <ul class="list-group">
@@ -130,7 +130,7 @@ Add badges to any list group item to show unread counts, activity, and more with
 
 ## Custom content
 
-Add nearly any HTML within, even for linked list groups like the one below, with the help of [flexbox utilities]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/flexbox/).
+Add nearly any HTML within, even for linked list groups like the one below, with the help of [flexbox utilities]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/flex/).
 
 {% example html %}
 <div class="list-group">

docs/4.0/components/navs.md

@@ -220,7 +220,7 @@ Similar to the `.nav-fill` example using a `<nav>`-based navigation, be sure to
 {% endexample %}
 ## Working with flex utilities
 
-If you need responsive nav variations, consider using a series of [flexbox utilities]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/flexbox/). While more verbose, these utilities offer greater customization across responsive breakpoints. In the example below, our nav will be stacked on the lowest breakpoint, then adapt to a horizontal layout that fills the available width starting from the small breakpoint.
+If you need responsive nav variations, consider using a series of [flexbox utilities]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/flex/). While more verbose, these utilities offer greater customization across responsive breakpoints. In the example below, our nav will be stacked on the lowest breakpoint, then adapt to a horizontal layout that fills the available width starting from the small breakpoint.
 
 {% example html %}
 <nav class="nav nav-pills flex-column flex-sm-row">

docs/4.0/components/pagination.md

@@ -135,7 +135,7 @@ Fancy larger or smaller pagination? Add `.pagination-lg` or `.pagination-sm` for
 
 ## Alignment
 
-Change the alignment of pagination components with [flexbox utilities]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/flexbox/).
+Change the alignment of pagination components with [flexbox utilities]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/flex/).
 
 {% example html %}
 <nav aria-label="Page navigation example">