icmp: fix typo.

Change-Id: I327897f8fa06e4f65dbd32b81be80fdc438bc6e0
Reviewed-on: https://go-review.googlesource.com/18656
Reviewed-by: David Symonds <dsymonds@golang.org>

icmp/ipv6.go

@@ -12,7 +12,7 @@ import (
 
 const ipv6PseudoHeaderLen = 2*net.IPv6len + 8
 
-// IPv6PseudoHeader returns an IPv6 pseudo header for checkusm
+// IPv6PseudoHeader returns an IPv6 pseudo header for checksum
 // calculation.
 func IPv6PseudoHeader(src, dst net.IP) []byte {
 	b := make([]byte, ipv6PseudoHeaderLen)