Clarify path for settings files with manual configuration.

The configuration files need to be placed in
`/opt/docker/zulip/zulip/settings/etc-zulip` so that Zulip will pick
them up properly. This commit clarifies that in the README.md file.

Author: eriktews

README.md

@@ -222,7 +222,7 @@ Zulip server's settings.
 An alternative approach is to set `MANUAL_CONFIGURATION: "True"` and
 `LINK_SETTINGS_TO_DATA: "True"` in `docker-compose.yml`.  If you do that, you
 can provide a `settings.py` file and a `zulip-secrets.conf` file in
-`/opt/docker/zulip/zulip/`, and the container will use those.
+`/opt/docker/zulip/zulip/settings/etc-zulip/`, and the container will use those.
 
 ### Starting the server