Fix incorrect operator example

Author: lucalves

data/reusables/github-actions/using-context-or-environment-variables.md

@@ -11,7 +11,7 @@ name: CI
 on: push
 jobs:
   prod-check:
-    if: github.ref == 'refs/heads/main'
+    if: ${{ github.ref == 'refs/heads/main' }}
     runs-on: ubuntu-latest
     steps:
       - run: echo "Deploying to production server on branch $GITHUB_REF"