Use "stack exec -- ..." more in GUIDE.md
Use "stack exec -- ..." more in GUIDE.md

This isn't a necessary change, but I think we should just encourage the
habit of always using "--".

MichielDerhaeg/stack,deech/stack,martin-kolinek/stack,AndreasPK/stack,anton-dessiatov/stack,Fuuzetsu/stack,juhp/stack,Blaisorblade/stack,mrkkrp/stack,AndrewRademacher/stack,Fuuzetsu/stack,Blaisorblade/stack,MichielDerhaeg/stack,AndreasPK/stack,deech/stack,anton-dessiatov/stack,martin-kolinek/stack,mrkkrp/stack,juhp/stack,AndrewRademacher/stack