Hi! In https://github.com/zed-extensions/ruby/issues/162 we renamed
embedded template languages:

- `HTML/ERB` to `HTML+ERB`
- `YAML/ERB` to `YAML+ERB`
- `JS/ERB` to `JS+ERB`

This pull request updates the Ruby extension documentation to reflect
that change. Thanks!

Release Notes:

- N/A