cluster: fix comment regarding child_process file

Fixed comment for the description of INTERNAL_PREFIX in child_process
whereas the value is present in the internal/child_process.js.

Author: yashLadha

lib/internal/cluster/utils.js

@@ -17,7 +17,7 @@ function sendHelper(proc, message, handle, cb) {
   if (!proc.connected)
     return false;
 
-  // Mark message as internal. See INTERNAL_PREFIX in lib/child_process.js
+  // Mark message as internal. See INTERNAL_PREFIX in lib/internal/child_process.js
   message = { cmd: 'NODE_CLUSTER', ...message, seq };
 
   if (typeof cb === 'function')