[NTG-context] correction

re: might be an error in ConTeXt file prop-mis.tex I had some difficulty with typesetting backgrounds. It seems there is an error in file prop-mis.tex (version 2004-06-21): token "\popsplitnegativeproperty" is never defined but "\popsplitoverprintproperty" is twice. I think the second one should be \popsplitnegativeproperty, because then the sequence \startbackground some text \stopbackground does typeset after regenerating the formats. Hope this helps. Hans van der Meer