Normalized the heading hierarchy in the documentation for the Button dropdowns component.

docs/_includes/components/button-dropdowns.html

@@ -8,7 +8,7 @@
     <p>Button dropdowns require the <a href="../javascript/#dropdowns">dropdown plugin</a> to be included in your version of Bootstrap.</p>
   </div>
 
-  <h3 id="btn-dropdowns-single">Single button dropdowns</h3>
+  <h2 id="btn-dropdowns-single">Single button dropdowns</h2>
   <p>Turn a button into a dropdown toggle with some basic markup changes.</p>
   <div class="bs-example" data-example-id="single-button-dropdown">
     <div class="btn-group">
@@ -88,7 +88,7 @@
 </div>
 {% endhighlight %}
 
-  <h3 id="btn-dropdowns-split">Split button dropdowns</h3>
+  <h2 id="btn-dropdowns-split">Split button dropdowns</h2>
   <p>Similarly, create split button dropdowns with the same markup changes, only with a separate button.</p>
   <div class="bs-example" data-example-id="split-button-dropdown">
     <div class="btn-group">
@@ -194,7 +194,7 @@
 </div>
 {% endhighlight %}
 
-  <h3 id="btn-dropdowns-sizing">Sizing</h3>
+  <h2 id="btn-dropdowns-sizing">Sizing</h2>
   <p>Button dropdowns work with buttons of all sizes.</p>
   <div class="bs-example" data-example-id="button-dropdown-sizing">
     <div class="btn-toolbar" role="toolbar">
@@ -272,7 +272,7 @@
 </div>
 {% endhighlight %}
 
-  <h3 id="btn-dropdowns-dropup">Dropup variation</h3>
+  <h2 id="btn-dropdowns-dropup">Dropup variation</h2>
   <p>Trigger dropdown menus above elements by adding <code>.dropup</code> to the parent.</p>
   <div class="bs-example" data-example-id="button-dropdown-dropup">
     <div class="btn-toolbar" role="toolbar">