Fix ruby-version of Linting GitHub Actions

Author: wataru86

.github/workflows/lint.yml

@@ -10,7 +10,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: 3.0
+          ruby-version: '3.0'
           bundler-cache: true # 'bundle install' and cache
       - name: Rubocop
         run: bundle exec rake rubocop