chruby: Fix typo nit and dotfiles in caveats.

Updates caveats to incorporate several commits to the chruby formula (https://github.com/postmodern/chruby/blob/master/homebrew/chruby.rb):

- Removes an errant single-quote typo in caveats.
- Uniformly recommends `~/.bashrc` or `~/.zshrc` in favor of `/etc/profile.d/chruby.sh` and `~/.bash_profile` in caveats.