doc: mark worker.id as integer in cluster docs

PR-URL: #42684
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Akhil Marsonya <akhil.marsonya27@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/cluster.md

@@ -477,7 +477,7 @@ worker.kill();
 added: v0.8.0
 -->
 
-* {number}
+* {integer}
 
 Each new worker is given its own unique id, this id is stored in the
 `id`.