Fix YAML formatting: quote ruby-head for consistency

Author: claude

.github/workflows/rspec.yml

@@ -5,7 +5,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ["2.7", "3.1", "3.2", "3.3", "3.4", "4.0", ruby-head]
+        ruby: ["2.7", "3.1", "3.2", "3.3", "3.4", "4.0", "ruby-head"]
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3