fix broken link on CBL tutorial

I'm following this tutorial:

https://docs.couchbase.com/tutorials/mobile-travel-sample/csharp/installation/docker.html#sync-gateway

It has this note:

 NOTE : If you are running the Sync Gateway in a docker container, please make sure that you have the Couchbase Server running in a container as well. If not, please follow instructions here to install the server container.

The here should link to section in that doc about installing docker but instead leads to the main doc page.  It should link to this:

https://docs.couchbase.com/tutorials/mobile-travel-sample/csharp/installation/docker.html#couchbase-server