doc: use UDPv4/UDPv6 consistently with TCPv4/TCPv6

(Arguably, the whole terminology is not perfect, but at least it will be
consistent with TCP now.)

PR-URL: #41824
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Mestery <mestery@protonmail.com>

doc/api/cluster.md

@@ -761,7 +761,7 @@ The `addressType` is one of:
 * `4` (TCPv4)
 * `6` (TCPv6)
 * `-1` (Unix domain socket)
-* `'udp4'` or `'udp6'` (UDP v4 or v6)
+* `'udp4'` or `'udp6'` (UDPv4 or UDPv6)
 
 ## Event: `'message'`