Fix csslint errors.

docs/examples/sticky-footer-navbar/index.html

@@ -71,7 +71,7 @@
       <p>Back to <a href="../sticky-footer">the default sticky footer</a> minus the navbar.</p>
     </div>
 
-    <div id="footer">
+    <div class="footer">
       <div class="container">
         <p class="text-muted">Place sticky footer content here.</p>
       </div>

docs/examples/sticky-footer-navbar/sticky-footer-navbar.css

@@ -8,7 +8,7 @@ body {
   /* Margin bottom by footer height */
   margin-bottom: 60px;
 }
-#footer {
+.footer {
   position: absolute;
   bottom: 0;
   width: 100%;
@@ -29,7 +29,7 @@ body > .container {
   margin: 20px 0;
 }
 
-#footer > .container {
+.footer > .container {
   padding-right: 15px;
   padding-left: 15px;
 }