ipv4: fix typo in Header.Parse documentation

Change-Id: I889a02587b39e448ccdc5d723183ab550104ec5b
Reviewed-on: https://go-review.googlesource.com/112816
Reviewed-by: Ian Lance Taylor <iant@golang.org>

ipv4/header.go

@@ -98,7 +98,7 @@ func (h *Header) Marshal() ([]byte, error) {
 	return b, nil
 }
 
-// Parse parses b as an IPv4 header and sotres the result in h.
+// Parse parses b as an IPv4 header and stores the result in h.
 func (h *Header) Parse(b []byte) error {
 	if h == nil || len(b) < HeaderLen {
 		return errHeaderTooShort