strings: fix nonexistent path in comment

There is a part in the comment that points to a non-existent file.
It seems to have been overlooked in following PR.
https://go-review.googlesource.com/c/go/+/98518/

Change-Id: I21dbfbd270c654d5cd7fa88d114a356862612d90
Reviewed-on: https://go-review.googlesource.com/c/go/+/210298
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: po3rin

src/strings/strings.go

@@ -1094,7 +1094,7 @@ func Index(s, substr string) int {
 		i++
 		fails++
 		if fails >= 4+i>>4 && i < t {
-			// See comment in ../bytes/bytes_generic.go.
+			// See comment in ../bytes/bytes.go.
 			j := indexRabinKarp(s[i:], substr)
 			if j < 0 {
 				return -1