Minor edit doc KhalimskySpaceND

It's seems like their is an issue with the documentation of function sCell of KhalimskySpaceND ( http://dgtal.org/doc/nightly/classDGtal_1_1KhalimskySpaceND.html#a1223cf533c27e6670e4c3455f27fdac1 ):

>   From the Khalimsky coordinates of a cell and a sign, builds the corresponding unsigned cell.

but returns a signed cell (SCell)

(See Issue https://github.com/DGtal-team/DGtal/issues/872 )