Minor fix of _cat output (#23211) (#23213)

One line was missing a trailing "\n"

core/src/main/java/org/elasticsearch/rest/action/cat/RestThreadPoolAction.java

@@ -60,7 +60,7 @@ public class RestThreadPoolAction extends AbstractCatAction {
     @Override
     protected void documentation(StringBuilder sb) {
         sb.append("/_cat/thread_pool\n");
-        sb.append("/_cat/thread_pool/{thread_pools}");
+        sb.append("/_cat/thread_pool/{thread_pools}\n");
     }
 
     @Override