Fix the comment in ecp_nistz256_ord_sqr_mont to match code and prototype.

The counter is accessed as x2, not w2, so this is a uint64_t parameter.

Change-Id: I97a5dabc521fc00fc366a67712bc4932b256532f
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/55145
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: Bob Beck <bbe@google.com>
Reviewed-by: Bob Beck <bbe@google.com>

crypto/fipsmodule/ec/asm/p256-armv8-asm.pl

@@ -1365,7 +1365,7 @@ $code.=<<___;
 
 ////////////////////////////////////////////////////////////////////////
 // void ecp_nistz256_ord_sqr_mont(uint64_t res[4], uint64_t a[4],
-//                                int rep);
+//                                uint64_t rep);
 .globl	ecp_nistz256_ord_sqr_mont
 .type	ecp_nistz256_ord_sqr_mont,%function
 .align	4