updated a docstring

Author: syed-gilani

astroquery/mast/services.py

@@ -169,7 +169,7 @@ def _request(self, method, url, params=None, data=None, headers=None,
         cache : bool
             Default False. Use of bulit in _request caching
         use_json: bool
-            Default False. if True then use the json parameter for passing data to super()._request
+            Default False. if True then data is already in json format.
 
         Returns
         -------