guide: update requirements when requesting a commit bit.

Using the GitHub username ensures we have one handle that is used across
GitHub, Trac and the mailing lists. Especially as the GitHub website
only displays usernames and not real names, it can get really confusing
and I would like to avoid introducing more ambiguous handles.

guide/xml/project.xml

@@ -1112,7 +1112,7 @@ To use the current port, you must be in a port's directory.</screen>
       </listitem>
 
       <listitem>
-        <para>the identity you'd like to use as a member of the project, a.k.a. the
+        <para>your github username. This will be used as the identity the
         "handle", as part of your <literal><replaceable>handle</replaceable>@macports.org</literal>
         alias.</para>
       </listitem>