Fix YAML syntax error.

Author: nusbaume

.github/workflows/fleximod_test.yaml

@@ -31,7 +31,7 @@ jobs:
           python-version: "3.12"
 
       - name: checkout git-fleximod
-        use: actions/checkout@v4
+        uses: actions/checkout@v4
         with:
           repository: ESMCI/git-fleximod
           ref: main