docs: example with required version

[felixhummel]

Not supplying the version results in the following error:

mise ERROR error parsing config file: ~/.config/mise/config.toml
mise ERROR TOML parse error at line 5, column 17
  |
5 | "ubi:cli/cli" = { exe = "gh" }
  |                 ^^^^^^^^^^^^^^
missing version

mise ERROR Run with --verbose or MISE_VERBOSE=1 for more information