`OpenApiSpex.Plug.SwaggerUI` does not expand `:path`/`:paths` w/ `Phoenix.Endpoint.path`

[petermueller]

While I was working on making a :path prefix for the endpoint of my Phoenix app able to configured, I noticed that the :path field passed to OpenApiSpex.Plug.SwaggerUI is not expanded using the endpoint module. This seemed like an oversight (and was breaking my setup).

I have a PR I will push in a bit