Change install docs to fix wrong command

I fixed sample command for changing permission of directory which is exposed
to docker container.

Current docs command us to change permissions of "rally_db" directory.
But this directory is never created and used to being exposed.

So I changed it from "rally_db" to "/var/lib/rally_container" which
is exposed to container.

Change-Id: I61ffb4ce2c5546678bb97e2052bb196811b3e0bf

doc/source/install.rst

@@ -148,7 +148,7 @@ be done by the following steps:
 .. code-block:: bash
 
    sudo mkdir /var/lib/rally_container
-   sudo chown 65500 rally_db
+   sudo chown 65500 /var/lib/rally_container
    docker run -it -v /var/lib/rally_container:/home/rally rallyforge/rally
 
 .. note::