runtime/internal/atomic: remove incorrect pointer indirection in comment

Change-Id: I9d743b7f6b001158299bea4af4aede678654bc8e
GitHub-Last-Rev: 7e07834
GitHub-Pull-Request: #46851
Reviewed-on: https://go-review.googlesource.com/c/go/+/329730
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ben Shi <powerman1st@163.com>

src/runtime/internal/atomic/atomic_386.s

@@ -65,7 +65,7 @@ TEXT ·Xaddint64(SB), NOSPLIT, $0-20
 
 // bool ·Cas64(uint64 *val, uint64 old, uint64 new)
 // Atomically:
-// if(*val == *old){
+// if(*val == old){
 // *val = new;
 // return 1;
 // } else {