Update environment to Rubygems.org.

Author: moroland

.github/workflows/gem-publish.yml

@@ -12,7 +12,7 @@ jobs:
       id-token: write # IMPORTANT: this permission is mandatory for trusted publishing
       contents: write # IMPORTANT: this permission is required for `rake release` to push the release tag
 
-    environment: RubyGems.org
+    environment: Rubygems.org
 
     steps:
       # Set up