Update asset_management.rst

I'm getting this message:

    The --watch option is deprecated. Please use the assetic:watch command instead.

Author: Geert Eltink

cookbook/assetic/asset_management.rst

@@ -545,7 +545,7 @@ command will automatically regenerate assets *as they change*:
 
 .. code-block:: bash
 
-    $ php app/console assetic:dump --watch
+    $ php app/console assetic:watch
 
 Since running this command in the ``dev`` environment may generate a bunch
 of files, it's usually a good idea to point your generated asset files to