Correct link in readme.
Correct link in readme.

Fixes https://github.com/gfx-rs/gfx-rs/issues/766
JohnColanduoni/gfx,gfx-rs/gfx-rs,aepsil0n/gfx-rs,tanderegg/gfx,sectopod/gfx-rs,red75prime/gfx,alexcrichton/gfx,mitchmindtree/gfx-rs,Bastacyclop/gfx,JohnColanduoni/gfx,ColinKinloch/gfx,bvssvni/gfx-rs,tanderegg/gfx,ColinKinloch/gfx,tempbottle/gfx,davll/gfx