Fix rbenv-installer links

[liamnichols]

The original links seem to 404 now, maybe a change from GitHub?

https://github.com/rbenv/rbenv-installer/raw/HEAD/bin/rbenv-installer
https://github.com/rbenv/rbenv-installer/raw/HEAD/bin/rbenv-doctor

[liamnichols]

Looks like it specifically relates to using HEAD in the URL:

$ curl -I https://github.com/rbenv/rbenv-installer/raw/HEAD/bin/rbenv-installer
HTTP/2 404 

Replacing it with main is a 302 redirect:

$ curl -I https://github.com/rbenv/rbenv-installer/raw/main/bin/rbenv-installer
HTTP/2 302 
server: GitHub.com
location: https://raw.githubusercontent.com/rbenv/rbenv-installer/main/bin/rbenv-installer

Which lead to using https://raw.githubusercontent.com in the PR