doc: fixed worker.id type

per: https://github.com/joyent/node/pull/25459

originally submitted by @sonnyp

Reviewed By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/2378

doc/api/cluster.markdown

@@ -384,7 +384,7 @@ it can be obtained using `cluster.worker`.
 
 ### worker.id
 
-* {String}
+* {Number}
 
 Each new worker is given its own unique id, this id is stored in the
 `id`.