Fixed small type errors in the README
Fixed small type errors in the README

purposeindustries/picturefill,patrickkettner/picturefill,albell/picturefill,mcanthony/picturefill,scottjehl/picturefill,chrismcandrew/picturefill,yubin-huang/picturefill,rockmandew/picturefill,yubin-huang/picturefill,patrickkettner/picturefill,yufi113/picturefill,mcanthony/picturefill,demateu/picturefill,guardian/picturefill,guardian/picturefill,footiealf/picturefill,yufi113/picturefill,ddeveloperr/picturefill,purposeindustries/picturefill,albell/picturefill,iancrowther/picturefill,samiadelgado/picturefill,nucliweb/picturefill,demateu/picturefill,samiadelgado/picturefill,footiealf/picturefill,scottjehl/picturefill,rockmandew/picturefill,iancrowther/picturefill,chrismcandrew/picturefill,ddeveloperr/picturefill,jansepar/picturefill