gfx docu links to wrong docu

[Richie765]

The gfx documentation at https://godoc.org/github.com/veandco/go-sdl2/gfx links to SDL1.2 version of gfx.

Outdated links: https://www.ferzkopp.net/Software/SDL_gfx-2.0/Docs/html/index.html

Correct link: https://www.ferzkopp.net/Software/SDL2_gfx/Docs/html/index.html

[veeableful]

Thanks for reporting, @Richie765! I have fixed it in the master branch.