Use default puma socket in example config

The unicorn replacement 'puma' uses a unix socket in the example config [1] instead of a tcp port.
Using the non-existing tcp port results in "Internal API unreachable" on
git operations.

[1] https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/puma.rb.example#L34

config.yml.example

@@ -13,7 +13,7 @@ user: git
 # only listen on a Unix domain socket. For Unix domain sockets use
 # "http+unix://<urlquoted-path-to-socket>", e.g.
 # "http+unix://%2Fpath%2Fto%2Fsocket"
-gitlab_url: "http://localhost:8080"
+gitlab_url: "http+unix://%2Fhome%2Fgit%2Fgitlab%2Ftmp%2Fsockets%2Fgitlab-workhorse.socket"
 
 # See installation.md#using-https for additional HTTPS configuration details.
 http_settings: