Fix workflow matrix

Author: Andreas Frömer

.github/workflows/continuous-integration.yml

@@ -67,7 +67,7 @@ jobs:
       fail-fast: false
       matrix:
         php-version: ["7.2", "7.3", "7.4", "8.0"]
-        operating-system: "ubuntu-latest"
+        operating-system: ["ubuntu-latest"]
         experimental: [false]
         include:
           - php: "8.0"