Update: Include <username> in docs.

This includes the <username> tag in the docs.
Closes: #2207

doc/config.html

@@ -166,6 +166,11 @@
     For providing a stream, a username/password has to be provided, and must
     match what icecast expects.
    </div>
+   <h4>username</h4>
+   <div class=indentedbox>
+    For providing a stream, a username/password has to be provided, and must
+    match what icecast expects. Default is "source".
+   </div>
    <h4>mount</h4>
    <div class=indentedbox>
     Mountpoints are used to identify a particular stream on a icecast server,