[Rd] Typo in polygon.Rd (PR#8605)

[vincent.goulet@act.ulaval.ca]

Hi,

Small typo in the description of argument 'col' of function polygon(): "(For 
back-compatibiility, ..." should obviously be "(For back-compatibility...".

The mistake was still there in 2.3.0 minutes ago...

Best,

-- 
  Vincent Goulet, Associate Professor
  École d'actuariat
  Université Laval, Québec 
  Vincent.Goulet at act.ulaval.ca   http://vgoulet.act.ulaval.ca