Key is being used in place of Version when creating a URL

https://github.com/henomis/quickchart-go/blob/7eed6dd90c19210e5d2e5bb7fa1f1f39ad6378d7/quickchart-go.go#L71

`qc.Version` isn't being used when creating a url for a chart. Looks like `qc.Key` was mistakenly used when setting the version. I've verified locally that I need to set `qc.Key` and `qc.Version` as the version I want in order to set the version, which doesn't seem like the expected/desired behavior.

Let me know if you'd like for me to PR. Looks like a tiny fix though.

Thanks!