fix typo about token access in auth.rst (#157)

Author: kelvintaywl

docs-src/auth.rst

@@ -130,7 +130,7 @@ user access token instead of the top-level access token granted to your applicat
 
     # Save the bot token to an environmental variable or to your data store
     # for later use
-    os.environ["SLACK_USER_TOKEN"] = auth_response['user_access_token']
+    os.environ["SLACK_USER_TOKEN"] = auth_response['access_token']
     os.environ["SLACK_BOT_TOKEN"] = auth_response['bot']['bot_access_token']
 
     # Don't forget to let the user know that auth has succeeded!