docs: Remove --cache-dir from github actions docs due to #8126 (#8450)
### Description
As I see it, since the default cache location is now
`<root-dir>/turbo/.cache` due to #8126, there's no longer a point in
specifying the --cache-dir=.turbo as it is redundant no?
### Testing Instructions
Run a GitHub actions with
https://turbo.build/repo/docs/guides/ci-vendors/github-actions#caching-with-github-actionscache
and see if it caches properly