Clarify task cancellation command (#23667)

Makes it explicit that the node_id has to be included when canceling a task.

docs/reference/cluster/tasks.asciidoc

@@ -165,9 +165,10 @@ If a long-running task supports cancellation, it can be cancelled by the followi
 
 [source,js]
 --------------------------------------------------
-POST _tasks/task_id:1/_cancel
+POST _tasks/node_id:task_id/_cancel
 --------------------------------------------------
 // CONSOLE
+// TEST[s/task_id/1/]
 
 The task cancellation command supports the same task selection parameters as the list tasks command, so multiple tasks
 can be cancelled at the same time. For example, the following command will cancel all reindex tasks running on the