Bug 1087913 - Update celery instructions on RTD after bug 1016117

docs/installation.rst

@@ -97,7 +97,7 @@ Running the ingestion tasks
 
   .. code-block:: bash
 
-     (venv)vagrant@precise32:~/treeherder-service$ python manage.py celery worker -B
+     (venv)vagrant@precise32:~/treeherder-service$ celery -A treeherder worker -B
 
   The "-B" option tells the celery worker to startup a beat service, so that periodic tasks can be executed.
   You only need one worker with the beat service enabled. Multiple beat services will result in periodic tasks being executed multiple times