chore: fix incorrect yaml in update-contributors.yml (#26850)

Yaml is incorrect causing the workflow to not run at all.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: mrgrain

.github/workflows/update-contributors.yml

@@ -22,7 +22,7 @@ jobs:
           # Git commit details
           branch: automation/update-contributors
           author: aws-cdk-automation <[email protected]>
-          commit-message: chore: update Contributors File
+          commit-message: 'chore: update Contributors File'
           # Pull Request details
           title: 'chore: update Contributors File'
           labels: auto-approve