Use correct JSON for the GitHub Action

.github/workflows/backport.yml

@@ -29,5 +29,5 @@ jobs:
              Backport of <%= mergeCommitSha %> from #<%= number %>.
           head_template: "backport/pr-<%= number %>-to-<%= base %>"
           label_pattern: "^backport/(?<base>([^ ]+))$"
-          labels_template: "[ backport ]"
+          labels_template: "[ 'backport' ]"
           title_template: "[<%= base %>] <%= title %>"