Fix await typo in quickstart doc

PR aio-libs#3649 by @eteamin

Author: eteamin

docs/client_quickstart.rst

@@ -178,7 +178,7 @@ Any of session's request methods like :func:`request`,
 `json` parameter::
 
   async with aiohttp.ClientSession() as session:
-      async with session.post(url, json={'test': 'object'})
+      await session.post(url, json={'test': 'object'})
 
 
 By default session uses python's standard :mod:`json` module for