Add button is-link html example (#1390)

docs/documentation/modifiers/syntax.html

@@ -92,6 +92,9 @@ doc-subtab: syntax
 <a class="button is-primary">
   Button
 </a>
+<a class="button is-link">
+  Button
+</a>
 <a class="button is-info">
   Button
 </a>