doc: correct grammar in cluster.markdown

per: https://github.com/joyent/node/pull/14352

originally submitted by @AlexKVal

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

doc/api/cluster.markdown

@@ -24,7 +24,7 @@ all share server ports.
       });
     } else {
       // Workers can share any TCP connection
-      // In this case its a HTTP server
+      // In this case it is an HTTP server
       http.createServer(function(req, res) {
         res.writeHead(200);
         res.end("hello world\n");