[trigger release] Finalize and fix syntax of prepare-workflow run (#762)

* test syntax fix

Author: nmuesch

.github/workflows/prepare-release.yml

@@ -10,7 +10,7 @@ jobs:
    # Use ubuntu-18.04 image so we have Java 8 + Maven 3 - https://github.com/actions/virtual-environments/blob/main/images/linux/Ubuntu1804-README.md
    # We only need to use the `mvn release:prepare` command here to perform the SCM actions, the full build and release happens in .gitlab-ci.yml
    runs-on: ubuntu-18.04
-   if: contains(github.event.head_commit.message, "[trigger release]")
+   if: contains(github.event.head_commit.message, '[trigger release]')
    steps:
    - name: Checkout code
      uses: actions/checkout@v2