runtime: fix comment typo

This was a typo mistake according to if cond and runtime/mheap.go:323

Change-Id: Id046d4afbfe0ea43cb29e1a9f400e1f130de221d
Reviewed-on: https://go-review.googlesource.com/102575Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mbitmap.go

@@ -257,7 +257,7 @@ func (s *mspan) objIndex(p uintptr) uintptr {
 		return 0
 	}
 	if s.baseMask != 0 {
-		// s.baseMask is 0, elemsize is a power of two, so shift by s.divShift
+		// s.baseMask is non-0, elemsize is a power of two, so shift by s.divShift
 		return byteOffset >> s.divShift
 	}
 	return uintptr(((uint64(byteOffset) >> s.divShift) * uint64(s.divMul)) >> s.divShift2)