ignore forum link in full link check (#4449)

markdown-link-check.full.json

         {
             "pattern": "https://github.com/Unity-Technologies/ml-agents/(tree|archive)/release_[0-9]+.*",
             "comment": "Allow future release tags"
+        },
+        {
+            "pattern": "https://forum.unity.com/forums/ml-agents.453/",
+            "comment": "Not reachable from github action"
         }
     ]
 }