Fix the usage example in the README
Fix the usage example in the README

Closes #712

tomaka/glutin,ozkriff/glutin,tomaka/winit,servo/glutin,mitchmindtree/glutin,vberger/glutin,nc4rrillo/glutin,ecoal95/glutin,mbrubeck/glutin,mbrubeck/glutin,bvssvni/glutin