Use .alert-heading instead of hardcoding <h4>

docs/javascript/alerts.md

@@ -22,7 +22,7 @@ When using a `.close` button, it must be the first child of the `.alert-dismissi
 {% example html %}
 <div class="alert alert-danger alert-dismissible fade in" role="alert">
   <button type="button" class="close" data-dismiss="alert"><span aria-hidden="true">&times;</span><span class="sr-only">Close</span></button>
-  <h4>Oh snap! You got an error!</h4>
+  <h4 class="alert-heading">Oh snap! You got an error!</h4>
   <p>Change this and that and try again. Duis mollis, est non commodo luctus, nisi erat porttitor ligula, eget lacinia odio sem nec elit. Cras mattis consectetur purus sit amet fermentum.</p>
   <p>
     <button type="button" class="btn btn-danger">Take this action</button>

scss/_alert.scss

@@ -13,9 +13,9 @@
   @include border-radius($alert-border-radius);
 
   // Headings for larger alerts
-  h4 {
+  .alert-heading {
     margin-top: 0;
-    // Specified for the h4 to prevent conflicts of changing $headings-color
+    // Specified to prevent conflicts of changing $headings-color
     color: inherit;
   }
   // Provide class for links that match alerts