fix: update json encoding in workflow (#209)

Author: andrewleith

.github/workflows/docker.yaml

@@ -92,5 +92,5 @@ jobs:
     - name: Notify Slack channel if this job failed
       if: ${{ failure() }}
       run: |
-        json="{'text':'<!here> CI is failing in <https://github.com/cds-snc/notification-document-download-api/|notification-document-download-api> !'}"
+        json='{"text":"<!here> CI is failing in <https://github.com/cds-snc/notification-document-download-api/|notification-document-download-api> !"}'
         curl -X POST -H 'Content-type: application/json' --data "$json"  ${{ secrets.SLACK_WEBHOOK }}