Docs CI: simplify `only` ref

.gitlab-ci.yml

@@ -48,8 +48,7 @@ docs-deploy:
   script:
     - curl -X POST -F"token=$CI_JOB_TOKEN" -F'ref=master' https://gitlab.com/api/v4/projects/15685485/trigger/pipeline
   only:
-    refs:
-      - develop
+    - develop
 
 # Supposed to fail when translations are outdated, instead always passes
 #