Fix artUserPaintCtx MEL documentation

[silenthell]

According to the MEL documentation -stampProfile can take the values:

"gaussian", "soft", "solid" and "square"

However "soft" triggers an error. Since at lease Maya 2014 "poly" is the correct value it seems:

artUserPaintCtx -edit -stampProfile "poly" "artUserPaintContext"; //<- correct

artUserPaintCtx -edit -stampProfile "soft" "artUserPaintContext"; //<- error

[tj.galda]

Good catch, we'll update the docs.  You're right, the options should be gaussian, poly, solid, square.