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

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>