fix typo

Author: ko1

.github/workflows/ruby.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: ['2.6`, '2.7', '3.0', 'head', 'debug']
+        ruby-version: ['2.6', '2.7', '3.0', 'head', 'debug']
 
     steps:
     - uses: actions/checkout@v2