Fix API path of `Repository.get_git_ref` (#2992)

Rationale:

> 1. Note that PyGithub queries "git/refs/..." in the API.
> 2. After back and forth with Github support, we found out that
"git/refs/...." is internally pointing to the "List matching references"
API call. -
https://docs.github.com/en/enterprise-server@3.11/rest/git/refs?apiVersion=2022-11-28#list-matching-references
- Whereas the "Get a ref" call is strictly limited to 'git/ref/...'
(Note the missing S).
However, PyGitHub uses it in the get_git_ref method.

This has caused a bug in my application where calling
`Repository.get_git_ref("foo")` would return me any reference matching
`foo` - probably as prefix - , rather the one and only reference `foo`.

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Co-authored-by: Enrico Minack <github@enrico.minack.dev>