runtime: fix out-of-date comment on gcWork usage

Change-Id: I3c21ffa80a5c14911e07238b1f64bec686ed7b72
Reviewed-on: https://go-review.googlesource.com/14980Reviewed-by: Minux Ma <minux@golang.org>

src/runtime/mgcwork.go

@@ -51,7 +51,7 @@ func (wp wbufptr) ptr() *workbuf {
 //     (preemption must be disabled)
 //     gcw := &getg().m.p.ptr().gcw
 //     .. call gcw.put() to produce and gcw.get() to consume ..
-//     if gcphase == _GCmarktermination {
+//     if gcBlackenPromptly {
 //         gcw.dispose()
 //     }
 //