fix matrix

Author: barryvdh

.github/workflows/run-tests.yml

@@ -13,7 +13,7 @@ jobs:
       matrix:
         php: [7.4, 7.3, 7.2, 7.1, 7.0]
         dependency-version: [prefer-lowest, prefer-stable]
-        os: ubuntu-latest
+        os: [ubuntu-latest]
 
     name: PHP${{ matrix.php }} - ${{ matrix.os }} - ${{ matrix.dependency-version }}