Correct nbtsplitloc.c comment.

Correct nbtsplitloc.c comment.

The logic just added by commit e3899ffd falls back on a 50:50 page split
in the event of a new item that's just to the right of our provisional
"many duplicates" split point.  Fix a comment that incorrectly claimed
that the new item had to be just to the left of our provisional split
point.

Backpatch: 12-, just like commit e3899ffd.