Fix inconsistency in components/modal docs (#499)

docs/documentation/components/modal.html

@@ -105,7 +105,7 @@ doc-subtab: modal
       <!-- Content ... -->
     </section>
     <footer class="modal-card-foot">
-      <a class="button is-primary">Save changes</a>
+      <a class="button is-success">Save changes</a>
       <a class="button">Cancel</a>
     </footer>
   </div>