DOC: in TreeTagger tagset for French, fix error(s)

1. DET:POS stands for “déterminant possessif (ma, ta…)” (or “adjectif possessif (ma, ta…)”, both are correct).
   The “pronom possessif” description corresponds only to the PRO:POS tag.
2. PRO:DEM stands for "possessif (pronom ou déterminant) (ce, celle, cette, cela, ça, tel...)".
3. PRO:IND stands for "pronom ou adjectif indéfini (tous, chacun, certaines, aucune...)".
   1. Actually, there are neither DET:DEM nor DET:IND tags (just DET:ART and DET:POS). The "article indéfini" category is included in DET:ART, not in PRO:IND, so "adjectif indéfini" must be distinguished from "déterminant indéfini".

These errors appear in the source document (the official tagset documentation) but should be corrected anyway, shouldn’t they ?

(from redmine: issue id 2789, created on 2020/03/20 by Benedicte Pincemin)