G1: fix incorrect region type documentation in HeapRegionType

In `class HeapRegionType`:

```
  // 01000 0 [16] Old Mask
  //
  // 10000 0 [32] Archive Mask
  // 11100 0 [56] Open Archive
  // 11100 1 [57] Closed Archive
  ...
    ArchiveMask = 32,
    OpenArchiveTag = ArchiveMask | PinnedMask,
    ClosedArchiveTag = ArchiveMask | PinnedMask + 1
```

Open/Closed Archive should have not Old Mask. The code is correct, but the documentation is incorrect.