Adjust two new date formats for Polish (pl)

* monthonly - happens to be the same as in English, but nonetheless
  copied explictly to avoid confusion in the future, as it's the only
  one to use the nominative form of month name instead of genitive

* pretty - needs to use genitive month name instead of nominative;
  same for dmy, mdy and ymd variants, as any form with different order
  would be gramatically awkward

I am a native Polish speaker.

Change-Id: Ic253f590b0a60196cf3d98ce8785b151bb576946