doc: fix travis stack download command
doc: fix travis stack download command

rvion/stack,rubik/stack,vigoo/stack,narrative/stack,martin-kolinek/stack,anton-dessiatov/stack,Fuuzetsu/stack,juhp/stack,vigoo/stack,AndrewRademacher/stack,lukexi/stack,lukexi/stack,Blaisorblade/stack,keent/stack,deech/stack,phadej/stack,achernyak/stack,Fuuzetsu/stack,harendra-kumar/stack,mrkkrp/stack,supermario/stack,Heather/stack,luigy/stack,anton-dessiatov/stack,harendra-kumar/stack,AndreasPK/stack,supermario/stack,AndreasPK/stack,narrative/stack,AndrewRademacher/stack,achernyak/stack,MichielDerhaeg/stack,keent/stack,mathhun/stack,martin-kolinek/stack,fizruk/stack,mrkkrp/stack,juhp/stack,Heather/stack,luigy/stack,sjakobi/stack,deech/stack,rvion/stack,Blaisorblade/stack,rubik/stack,MichielDerhaeg/stack,sjakobi/stack,fizruk/stack,mathhun/stack,phadej/stack