In order to include people who do not fit into the gender binary, gender fields really should be a free-form text field. As such, this patch changes the `choices` example to use a billing plan instead.

# References

* http://gender.wikia.com/wiki/Non-binary
* https://en.wikipedia.org/wiki/Genderqueer
* https://modelviewculture.com/pieces/the-argument-for-free-form-input
* https://www.facebook.com/