Change breakpoint-max implementation

- The `media-breakpoint-down()` uses the breakpoint itself instead of the next breakpoint. Use `media-breakpoint-down(lg)` instead of `media-breakpoint-down(md)` to target viewports smaller than the `lg` breakpoint.
- The `media-breakpoint-between()` mixin's second parameter also uses the breakpoint itself instead of the next breakpoint. Use `media-between(sm, lg)` instead of `media-breakpoint-between(sm, md)` to target viewports between the `sm` and `lg` breakpoints.

site/assets/scss/_subnav.scss

@@ -26,7 +26,7 @@
 }
 
 .bd-search {
-  @include media-breakpoint-down(sm) {
+  @include media-breakpoint-down(md) {
     width: 100%;
   }