net/netip: fix Addr.Unmap reference in Addr.Is4 godoc comment

In CL 339309 this was probably copied from the respective godoc comment
in package inet.af/netaddr, also see
https://pkg.go.dev/inet.af/netaddr#IP.Is4

In net/netip the type is named Addr, so adjust the godoc comment
accordingly.

Change-Id: Ib5ab8054067f8b74119efa4732192a8407189f9b
Reviewed-on: https://go-review.googlesource.com/c/go/+/401394
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/net/netip/netip.go

diff --git a/src/net/netip/netip.go b/src/net/netip/netip.go
index 8fad25d..7d8b203 100644
--- a/src/net/netip/netip.go
+++ b/src/net/netip/netip.go
@@ -456,7 +456,7 @@
 
 // Is4 reports whether ip is an IPv4 address.
 //
-// It returns false for IPv4-mapped IPv6 addresses. See IP.Unmap.
+// It returns false for IPv4-mapped IPv6 addresses. See Addr.Unmap.
 func (ip Addr) Is4() bool {
 	return ip.z == z4
 }