Fix incorrect comments in tuplesort.c

Hi,

The incorrect comment:

```
/*
* Initial size of memtuples array. We're trying to select this size so that
* array doesn't exceed ALLOCSET_SEPARATE_THRESHOLD and so that the overhead of
* allocation might possibly be lowered. However, we don't consider array sizes
* less than 1024.
*
*/
#define INITIAL_MEMTUPSIZE Max(1024, \
ALLOCSET_SEPARATE_THRESHOLD / sizeof(SortTuple) + 1)
```

The "doesn't exceed" are contrary to the macro. Maybe "slightly exceed"?

Attach a small patch. (also remove the empty line in this comment)

--
Regards,
ChangAo Chen