Update contributing file reference
Update contributing file reference

oxyplot/oxyplot,Jonarw/oxyplot,Jofta/oxyplot,HermanEldering/oxyplot,ze-pequeno/oxyplot,zur003/oxyplot,Kaplas80/oxyplot,olegtarasov/oxyplot,objorke/oxyplot,olegtarasov/oxyplot,NilesDavis/oxyplot,objorke/oxyplot,HermanEldering/oxyplot,Isolocis/oxyplot,objorke/oxyplot,Kaplas80/oxyplot