net/netcheck: fix probeProto.String result for IPv6 probes

This bug was introduced in e6b84f215 (May 2020) but was only used in
tests when stringifying probeProto values on failure so it wasn't
noticed for a long time.

But then it was moved into non-test code in 8450a18aa (Jun 2024) and I
didn't notice during the code movement that it was wrong. It's still
only used in failure paths in logs, but having wrong/ambiguous
debugging information isn't the best.

Whoops.

Updates tailscale/corp#20654

Change-Id: I296c727ed1c292a04db7b46ecc05c07fc1abc774
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/netcheck/netcheck.go

@@ -345,7 +345,7 @@ func (p probeProto) String() string {
 	case probeIPv4:
 		return "v4"
 	case probeIPv6:
-		return "v4"
+		return "v6"
 	case probeHTTPS:
 		return "https"
 	}