Fix the document of external ceph for gnocchi

Change-Id: I75955012a839e52281e9a409eeab4a2c8d778cd2
Signed-off-by: ZijianGuo <guozijn@gmail.com>

doc/source/reference/storage/external-ceph-guide.rst

@@ -228,7 +228,7 @@ configurations:
 Gnocchi
 -------
 
-Modify ``/etc/kolla/config/gnocchi/gnocchi.conf`` file according to
+Modify ``/etc/kolla/config/gnocchi.conf`` file according to
 the following configuration:
 
 .. code-block:: ini