mergify: fix label do not merge, fixes failure with action

Author: 0xc0170

.mergify.yml

@@ -164,7 +164,7 @@ pull_request_rules:
     actions:
       label:
         add:
-          - do not merge
+          - "do not merge"
 
   # Check if version label is applied
   - name: release version is a must for merged PRs