Update README URLs based on HTTP redirects

[ReadmeCritic]

Created with https://github.com/dkhamsing/frankenstein

GitHub Corrected URLs

Was	Now
https://github.com/sstephenson/rbenv	https://github.com/rbenv/rbenv
https://github.com/sstephenson/rbenv-vars	https://github.com/rbenv/rbenv-vars
https://github.com/sstephenson/rbenv/issues	https://github.com/rbenv/rbenv/issues
https://github.com/sstephenson/rbenv/wiki/Authoring-plugins#rbenv-hooks	https://github.com/rbenv/rbenv/wiki/Authoring-plugins#rbenv-hooks
https://github.com/sstephenson/rbenv/wiki/Plugins	https://github.com/rbenv/rbenv/wiki/Plugins
https://github.com/sstephenson/rbenv/wiki/Understanding-binstubs	https://github.com/rbenv/rbenv/wiki/Understanding-binstubs
https://github.com/sstephenson/rbenv/wiki/Why-rbenv%3F	https://github.com/rbenv/rbenv/wiki/Why-rbenv%3F
https://github.com/sstephenson/ruby-build#readme	https://github.com/rbenv/ruby-build#readme

[mislav]

I was meaning to do this myself as part of the org transition, but thanks, robot! 👍

[ReadmeCritic]

😄