internal/bytealg: fix function reference in comments

There's no IndexShortStr func, refer to Index instead.

Change-Id: I6923e7ad3e910e4b5fb0c07d6339ddfec4111f4f
Reviewed-on: https://go-review.googlesource.com/c/go/+/170124
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/internal/bytealg/index_arm64.go

@@ -4,7 +4,7 @@
 
 package bytealg
 
-// Empirical data shows that using IndexShortStr can get better
+// Empirical data shows that using Index can get better
 // performance when len(s) <= 16.
 const MaxBruteForce = 16
 
@@ -14,7 +14,7 @@ func init() {
 }
 
 // Cutover reports the number of failures of IndexByte we should tolerate
-// before switching over to IndexShortStr.
+// before switching over to Index.
 // n is the number of bytes processed so far.
 // See the bytes.Index implementation for details.
 func Cutover(n int) int {