Align the path of config file with the book

In the book, the config file is registered like below:

    $ kubectl create cm --from-file ghost-config.js ghost-config

14-4-ghost.yaml

@@ -18,7 +18,7 @@ spec:
         command:
         - sh
         - -c
-        - cp /ghost-config/config.js /var/lib/ghost/config.js
+        - cp /ghost-config/ghost-config.js /var/lib/ghost/config.js
           && /entrypoint.sh npm start
         volumeMounts:
         - mountPath: /ghost-config