mention syntax reference

Author: gcb

help/tutorial/index.html

@@ -57,7 +57,7 @@ <h2>Introduction</h2>
                             <a href="02-emphasis.html" class="button button-primary button-next-lesson noselect">Begin Lesson</a>
                         </p>
                         <p>
-                            <a href="../" class="button button-primary button-previous noselect">What is Markdown?</a>
+                            <a href="../" class="button button-primary button-previous noselect">What is Markdown? (with basic CommonMark syntax reference)</a>
                         </p>
                     </div>
                 </div>