net: use IPv4 reserved address blocks for documentation

Updates #15228.

Change-Id: Iefdffa146703ee1c04afc2b71d9de1f0a0811f86
Reviewed-on: https://go-review.googlesource.com/32146
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/net.go

@@ -108,7 +108,7 @@ func init() {
 // and meaning of the strings is up to the implementation.
 type Addr interface {
 	Network() string // name of the network (for example, "tcp", "udp")
-	String() string  // string form of address (for example, "127.0.0.1:25", "[2001:db8::1]:80")
+	String() string  // string form of address (for example, "192.0.2.1:25", "[2001:db8::1]:80")
 }
 
 // Conn is a generic stream-oriented network connection.