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`.
Merge pull request #670 from alphagov/en-dash-beta

Use an en dash in the beta label