doc: fix missing argument for dns.resolvePtr()

PR-URL: https://github.com/nodejs/node/pull/12256
Fixes: https://github.com/nodejs/node/issues/12161
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/dns.md

@@ -323,7 +323,7 @@ Uses the DNS protocol to resolve name server records (`NS` records) for the
 contain an array of name server records available for `hostname`
 (e.g. `['ns1.example.com', 'ns2.example.com']`).
 
-## dns.resolvePtr(hostname)
+## dns.resolvePtr(hostname, callback)
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