Fix code block formatting in README (#164)

Author: artemrys

README.rst

@@ -99,14 +99,16 @@ You can also retrieve it by exporting environment variables.
 
 * Required for OAuth2 Installed Application Flow
 
-.. code-block::bash
+.. code-block:: bash
 
   export GOOGLE_ADS_CLIENT_ID=INSERT_OAUTH2_CLIENT_ID_HERE
   export GOOGLE_ADS_CLIENT_SECRET=INSERT_OAUTH2_CLIENT_SECRET_HERE
   export GOOGLE_ADS_REFRESH_TOKEN=INSERT_REFRESH_TOKEN_HERE
 
 * Required for OAuth2 Service Account Flow:
 
+.. code-block:: bash
+
   export GOOGLE_ADS_PATH_TO_PRIVATE_KEY_FILE=INSERT_PRIVATE_KEY_PATH_HERE
   export GOOGLE_ADS_DELEGATED_ACCOUNT=INSERT_DELEGATED_ACCOUNT_HERE