runtime: fix comment for gcAssistAlloc

Change-Id: I312e56e95d8ef8ae036d16444ab1e2df1285845d
Reviewed-on: https://go-review.googlesource.com/15403Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/mgcmark.go

@@ -202,7 +202,7 @@ func markrootSpans(gcw *gcWork, shard int) {
 // allowAssist is true, may assist GC scanning in proportion to the
 // allocations performed by this mutator since the last assist.
 //
-// It should only be called if gcAssistAlloc != 0.
+// It should only be called if gcBlackenEnabled != 0.
 //
 // This must be called with preemption disabled.
 //go:nowritebarrier