Changed (what) to (how) on line 75 (#1706)

docs/documentation/modifiers/responsive-helpers.html

@@ -72,7 +72,7 @@ doc-subtab: responsive-helpers
         <li><code>inline-block</code></li>
         <li><code>inline-flex</code></li>
       </ul>
-      <p>For example, here's what the <code>is-flex</code> helper works:</p>
+      <p>For example, here's how the <code>is-flex</code> helper works:</p>
     </div>
 
     <table class="table is-bordered">