`badge_github_version`: change `pkg` arg to `ref`

[bschilder]

Hi again,

This is a minor thing, but I found it about confusing until I looked into the code.

badge_github_version take the argument pkg:

pkg | package. If NULL (the default) the package is determined via the DESCRIPTION file.

but I think it actually means something closer to ref, as implemented in functions like badger::badge_github_actions:

## produces wrong link
badger::badge_github_version(pkg="rworkflows")

## produces correct link
badger::badge_github_version(pkg="neurogenomics/rworkflows")

If you agree, should be a simple fix to change the arg name to ref, and update the argument description accordingly.