Fix bugs caught by compiler warnings

[win93]

In two places there seems to be a typo that my compiler uncovered via -Wall. This pull request fixes these two mistakes:

• SDL2::Cursor::warp_globally is documented as having 2 parameters, but is rb_defined to have 3. Indeed the C function Cursor_s_warp_globally takes 2 parameters (excluding self).
  • SDL2::Surface::unset_color_key calls the C function Surface_set_color_key (again with the wrong arity). I assume the C function for unset was what was intended.

[ohai]

Looks good. Thank you for your contribution.