Use an en dash in the beta label.

For consistency with the [component from `static`](https://github.com/alphagov/static/commit/c8e240491497).

This is a quick fix, since I don’t have a lot of context around this partial as compared to the component, so I don't know whether these are intentionally separate implementations, or whether we should be updating this code to use the component from [`static`](https://github.com/alphagov/static).