[ML] Change docs test mute comment

The original comment mentioned issue elastic#48583, but issue elastic#48941
is specifically open for this mute.  However, this is
inappropriate, as the underlying reason the test cannot be
unmuted is the same as for all the other tests skipped with the
comment "Kibana sample data": issues elastic#51572, elastic#51576 and elastic#51678.

Closes elastic#48941

Author: droberts195

docs/reference/ml/anomaly-detection/apis/close-job.asciidoc

@@ -92,7 +92,7 @@ include::{es-repo-dir}/ml/ml-shared.asciidoc[tag=allow-no-jobs]
 --------------------------------------------------
 POST _ml/anomaly_detectors/low_request_rate/_close
 --------------------------------------------------
-// TEST[skip:sometimes fails due to https://github.com/elastic/elasticsearch/pull/48583#issuecomment-552991325]
+// TEST[skip:Kibana sample data]
 
 When the job is closed, you receive the following results: