Change shard allocation filter property and api (elastic#42602)

The current example is not working and a bit confused. This change tries
to match it with the sample of the watcher blog.

x-pack/docs/en/watcher/how-watcher-works.asciidoc

@@ -149,14 +149,14 @@ primary and all replicas of this particular shard will reload.
 Because the watches are executed on the node, where the watch shards are, you can create
 dedicated watcher nodes by using shard allocation filtering.
 
-You could configure nodes with a dedicated `node.attr.watcher: true` property and
+You could configure nodes with a dedicated `node.attr.role: watcher` property and
 then configure the `.watches` index like this:
 
 [source,js]
 ------------------------
 PUT .watches/_settings
 {
-  "index.routing.allocation.include": "watcher"
+  "index.routing.allocation.include.role": "watcher"
 }
 ------------------------
 // CONSOLE
@@ -442,4 +442,4 @@ references the `email_notification_subject` template:
   }
 }
 ----------------------------------------------------------------------
-// NOTCONSOLE
+// NOTCONSOLE