ruby docs: improve the example

- missing cd command
- invoke bundler through nix-shell, so it doesn't need to be on $PATH
  Note: running bundix through nix-shell won't work ATM, as the shell sets
  SSL_CERT_FILE=/no-cert-file.crt which prevents fetching throug https.
- use version from gemset to simplify updating
- don't break line in meta.description