doc: fix return type of server.address()

PR-URL: #22043
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/net.md

@@ -112,7 +112,7 @@ Emitted when the server has been bound after calling [`server.listen()`][].
 added: v0.1.90
 -->
 
-* Returns: {Object}
+* Returns: {Object|string}
 
 Returns the bound `address`, the address `family` name, and `port` of the server
 as reported by the operating system if listening on an IP socket