Fix booth_recode_w5 comment.

The file is util-64.c in BoringSSL.

Change-Id: I51891103254ae1541ea4c30f92c41d5d47c2ba55
Reviewed-on: https://boringssl-review.googlesource.com/12223
Reviewed-by: Adam Langley <agl@google.com>

crypto/ec/p256-x86_64.c

@@ -50,7 +50,7 @@ static const BN_ULONG ONE[P256_LIMBS] = {
 /* Precomputed tables for the default generator */
 #include "p256-x86_64-table.h"
 
-/* Recode window to a signed digit, see ecp_nistputil.c for details */
+/* Recode window to a signed digit, see util-64.c for details */
 static unsigned booth_recode_w5(unsigned in) {
   unsigned s, d;