icmp: fix a typo

Change-Id: Icd29cd883dab0c916e2285d6053074e97771b840
Reviewed-on: https://go-review.googlesource.com/21833
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

icmp/endpoint.go

@@ -51,7 +51,7 @@ func (c *PacketConn) ReadFrom(b []byte) (int, net.Addr, error) {
 	}
 	// Please be informed that ipv4.NewPacketConn enables
 	// IP_STRIPHDR option by default on Darwin.
-	// See golang.org/issue/9395 for futher information.
+	// See golang.org/issue/9395 for further information.
 	if runtime.GOOS == "darwin" && c.p4 != nil {
 		n, _, peer, err := c.p4.ReadFrom(b)
 		return n, peer, err