fix: wrapped string into slice to pass into List func

[Spuxy]

based on this example, there is an error, but it expects slice than simple string

func (r *ChannelsService) List(part []string) *ChannelsListCall {
	c := &ChannelsListCall{s: r.s, urlParams_: make(gensupport.URLParams)}
	c.urlParams_.SetMulti("part", append([]string{}, part...))
	return c
}

func accepts slice of string, but the example wants to pass just an string.

channelsListByUsername

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