Reinstate underscore in fields param, as per 40842fe8bf (commitcomment-21183519)

docs/advanced_topics/api/v2/usage.rst

@@ -424,7 +424,7 @@ the URL. For example:
 All exported fields will be returned in the response by default. You can use the
 ``?fields`` parameter to customise which fields are shown.
 
-For example: ``/api/v2/pages/1/?fields=title,body`` will return just the
+For example: ``/api/v2/pages/1/?fields=_,title,body`` will return just the
 ``title`` and ``body`` of the page with the id of 1.
 
 Default endpoint fields