spec: Changed user gesture error from SecurityError to NotAllowedError.

Minor breaking change, as discussed on #45. Closes #45.

Author: mgiuca

index.html

@@ -149,8 +149,8 @@ <h4>
             activation</a>, <a data-cite=
             "!promises-guide#reject-promise">reject</a> <var>p</var> with
             <a data-cite=
-            "!WEBIDL#securityerror"><code>SecurityError</code></a>, and abort
-            these steps.
+            "!WEBIDL#notallowederror"><code>NotAllowedError</code></a>, and
+            abort these steps.
             </li>
             <li>
               <a data-cite="!HTML#in-parallel">In parallel</a>: