Update celery.rst

With the latest version of Celery (3.1.6), following this tutorial produces the following error when attempting to start the celery worker: 

    user_preload = tuple(self.app.user_options['preload'] or ())
AttributeError: 'Flask' object has no attribute 'user_options'

Using `app` as the variable name here confuses celery. Renaming `app` to `flask_app` in the tutorial solves the issue and allows the celery worker to start successfully.

docs/patterns/celery.rst

@@ -60,12 +60,12 @@ Flask::
 
     from flask import Flask
 
-    app = Flask(__name__)
-    app.config.update(
+    flask_app = Flask(__name__)
+    flask_app.config.update(
         CELERY_BROKER_URL='redis://localhost:6379',
         CELERY_RESULT_BACKEND='redis://localhost:6379'
     )
-    celery = make_celery(app)
+    celery = make_celery(flask_app)
 
 
     @celery.task()