readme.md: example code does not compile

[meling]

./iago_test.go:16:34: not enough arguments in call to iagotest.CreateSSHGroup
	have (*testing.T, number)
	want (*testing.T, int, bool)
./iago_test.go:18:8: not enough arguments in call to g.Run
	have (unknown type)
	want (string, func(ctx context.Context, host iago.Host) error)
./iago_test.go:18:13: undefined: iago.Task
./iago_test.go:21:15: undefined: iago.P
./iago_test.go:22:15: undefined: iago.P
./iago_test.go:23:4: unknown field Mode in struct literal of type iago.Download