add a question about account changes to the help page

lms/templates/static_templates/help.html

@@ -298,6 +298,12 @@
             <p>There’s no need to delete you account. An old, unused edX account with no course completions associated with it will disappear.</p>
           </div>
         </article>
+        <article class="response">
+          <h3 class="question">I made a mistake creating my username – how do I fix it?</h3>
+          <div class ="answer" id="other_help_faq_answer_4.5">
+            <p>In most cases it would simplest and fastest to create a new account.  Your old unused account will vanish naturally. If you were not aware of your mistake until much later, you should send us a detailed change request and we will do our best to edit your username.  Please bear in mind that usernames are unique, and the one you want may be taken.</p>
+          </div>
+        </article>
         <article class="response">
           <h3 class="question">I am experiencing problems with the display. E.g., There are tools missing from the course display, or I am unable to view video.</h3>
           <div class ="answer" id="other_help_faq_answer_5">