bytes: fix typo and resolve to be less idiotic next time

R=bradfitz
CC=golang-dev
https://golang.org/cl/5340044

src/pkg/bytes/bytes.go

@@ -89,7 +89,7 @@ func Count(s, sep []byte) int {
 }
 
 // Contains returns whether subslice is within b.
-func Contains(b, subslice []string) bool {
+func Contains(b, subslice []byte) bool {
 	return Index(b, subslice) != -1
 }