Fix variable_values spelling in docs (#226)

Author: wilbertom

docs/usage/file_upload.rst

@@ -92,7 +92,7 @@ See `Streaming uploads on aiohttp docs`_.
 
 
 In order to stream local files, instead of providing opened files to the
-`variables_values` argument of `execute`, you need to provide an async generator
+`variable_values` argument of `execute`, you need to provide an async generator
 which will provide parts of the files.
 
 You can use `aiofiles`_

gql/transport/aiohttp.py

@@ -128,7 +128,7 @@ async def execute(
         :code:`execute` on a client or a session.
 
         :param document: the parsed GraphQL request
-        :param variables_values: An optional Dict of variable values
+        :param variable_values: An optional Dict of variable values
         :param operation_name: An optional Operation name for the request
         :param extra_args: additional arguments to send to the aiohttp post method
         :param upload_files: Set to True if you want to put files in the variable values