bytes: fixed typo.

R=r
CC=golang-dev
https://golang.org/cl/6301096

src/pkg/bytes/bytes.go

@@ -415,7 +415,7 @@ func Repeat(b []byte, count int) []byte {
 // ToUpper returns a copy of the byte array s with all Unicode letters mapped to their upper case.
 func ToUpper(s []byte) []byte { return Map(unicode.ToUpper, s) }
 
-// ToUpper returns a copy of the byte array s with all Unicode letters mapped to their lower case.
+// ToLower returns a copy of the byte array s with all Unicode letters mapped to their lower case.
 func ToLower(s []byte) []byte { return Map(unicode.ToLower, s) }
 
 // ToTitle returns a copy of the byte array s with all Unicode letters mapped to their title case.