Tree is second argument to geef_index_read_tree

[kjg]

Currently calling geef_index.read_tree(index, tree) raises an Argument Error, it looks like this is due to using the first argument twice in the underlying c code:

geef/c_src/index.c

Lines 83 to 87 in 9e9aa23

	if (!enif_get_resource(env, argv[0], geef_index_type, (void **) &index))
	return enif_make_badarg(env);
	if (!enif_get_resource(env, argv[0], geef_object_type, (void **) &tree))
	return enif_make_badarg(env);

This PR updates the second call to enif_get_resource to use the second argument (argv[0])

I got to here by following the call stack below:

geef/src/geef_index.erl

Line 123 in 9e9aa23

Reply = geef_nif:index_read_tree(Handle, TreeHandle),

geef/src/geef_nif.erl

Line 180 in 9e9aa23

index_read_tree(_Handle, _TreeHandle) ->