Fix small syntax issue in navbar doc (#843)

This isn't a major issue but it does break any HTML that it's copy/pasted into

docs/documentation/components/navbar.html

@@ -111,7 +111,7 @@ doc-subtab: navbar
 <div class="navbar-item">
   <div class="field is-grouped">
     <p class="control">
-      <a id="class="button">
+      <a class="button">
         <span class="icon">
           <i class="fa fa-twitter"></i>
         </span>