Fixed a typo after porting _simplify_slices to C++.
Fixed a typo after porting _simplify_slices to C++.

prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r