Incorrect comments for ParallelCompactData::summarize_dense_prefix

For `ParallelCompactData::summarize_dense_prefix(HeapWord* beg, HeapWord* end)`, the comment says `beg` should be region aligned but not `end`. However, the method body starts with asserting both need to be region aligned.

```
  assert(region_offset(beg) == 0, "not RegionSize aligned");
  assert(region_offset(end) == 0, "not RegionSize aligned");
```