Add comment about ruby/setup-ruby@v1

The same comment can be found in the actions/starter-workflows workflow for Ruby: https://github.com/actions/starter-workflows/blob/2e489c26196ef3b0c1ff38e460beae83e522e2d4/ci/ruby.yml#L27-L29

Author: dentarg

content/actions/automating-builds-and-tests/building-and-testing-ruby.md

@@ -54,6 +54,9 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - name: Set up Ruby
+      # To automatically get bug fixes and new Ruby versions for ruby/setup-ruby,
+      # change this to (see https://github.com/ruby/setup-ruby#versioning):
+      # uses: ruby/setup-ruby@v1
         uses: ruby/setup-ruby@359bebbc29cbe6c87da6bc9ea3bc930432750108
         with:
           ruby-version: '3.1'