Rename the placeholder for contact adding

to make it clearer what a JID should look like.

converse.js

@@ -1674,7 +1674,7 @@
                     });
                 } else {
                     markup = converse.templates.add_contact_form({
-                        label_contact_username: __('Contact username'),
+                        label_contact_username: __('e.g. user@example.com'),
                         label_add: __('Add')
                     });
                 }