fix text-dy doc

The logich was literally upside down.

See dy in 3.x from this pull request I just made:
https://github.com/mapnik/mapnik/pull/2445

dy in 2.3.x (vertical-alignment=top has no effect here, can be ignored):
https://github.com/mapnik/mapnik/blob/2.3.x/tests/visual_tests/images/charspacing-lines-300-300-1.0-agg-reference.png
https://github.com/mapnik/mapnik/blob/2.3.x/tests/visual_tests/styles/charspacing-lines.xml