runtime: fix another out of date comment in GC

gcDrain used to be passed a *workbuf to start draining from, but now
it takes a gcWork, which hides whether or not there's an initial
workbuf. Update the comment to match this.

Change-Id: I976b58e5bfebc451cfd4fa75e770113067b5cc07
Reviewed-on: https://go-review.googlesource.com/8246Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgcmark.go

@@ -335,7 +335,7 @@ func scanframeworker(frame *stkframe, unused unsafe.Pointer, gcw *gcWork) {
 	}
 }
 
-// gcDrain scans objects in work buffers (starting with wbuf), blackening grey
+// gcDrain scans objects in work buffers, blackening grey
 // objects until all work buffers have been drained.
 //go:nowritebarrier
 func gcDrain(gcw *gcWork) {