Add missing opening single-quote

Author: stevegrunwell

.github/workflows/unit-tests.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         operating-system: [ubuntu-latest]
-        php-versions: ['5.6', '7.0', 7.1', '7.2', '7.3', '7.4']
+        php-versions: ['5.6', '7.0', '7.1', '7.2', '7.3', '7.4']
     name: PHP ${{ matrix.php-versions }}
 
     steps: