runtime: fix incorrect comment

This comment is probably a hold-over from when the heap bitmap was
interleaved and the shift was 0, 2, 4, or 6. Now the shift is 0, 1, 2,
or 3.

Change-Id: I096ec729e1ca31b708455c98b573dd961d16aaee
Reviewed-on: https://go-review.googlesource.com/18531
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mbitmap.go

@@ -333,7 +333,7 @@ func (h heapBits) hasPointers(size uintptr) bool {
 		return true
 	}
 	// Otherwise, at least a 2-word object, and at least 2-word aligned,
-	// so h.shift is either 0 or 4, so we know we can get the bits for the
+	// so h.shift is either 0 or 2, so we know we can get the bits for the
 	// first two words out of *h.bitp.
 	// If either of the first two words is a pointer, not pointer free.
 	b := uint32(*h.bitp >> h.shift)