fix MongoDB shell syntax highlighting

Author: xabbuh

cookbook/doctrine/mongodb_session_storage.rst

@@ -162,10 +162,10 @@ Because MongoDB uses dynamic collection schemas, you do not need to do anything
 session collection. However, you may want to add an index to improve garbage collection performance.
 From the `MongoDB shell`_:
 
-.. code-block:: text
+.. code-block:: bash
 
-    use session_db
-    db.session.ensureIndex( { "expires_at": 1 }, { expireAfterSeconds: 0 } )
+    $ use session_db
+    $ db.session.ensureIndex( { "expires_at": 1 }, { expireAfterSeconds: 0 } )
 
 .. _installed and configured a MongoDB server: http://docs.mongodb.org/manual/installation/
 .. _MongoDB shell: http://docs.mongodb.org/v2.2/tutorial/getting-started-with-the-mongo-shell/