doc: changing the documentation to reflect the change

Fixed the documentaion to reflect the changes in the default value
of worker.exitedAfterDisconnect

Fixes: nodejs#28837
Refs: nodejs#3743

doc/api/cluster.md

@@ -334,7 +334,7 @@ added: v6.0.0
 
 * {boolean}
 
-Set by calling `.kill()` or `.disconnect()`. Until then, it is `undefined`.
+Set by calling `.kill()` or `.disconnect()`. Until then, it is `false`.
 
 The boolean [`worker.exitedAfterDisconnect`][] allows distinguishing between
 voluntary and accidental exit, the master may choose not to respawn a worker