Bugfix for Saturn to Giraffe guide manifesting as issue in bundled app

[jwthomson]

Issue #362

This fix swaps out app.UseStaticFiles() for app.UseFileServer().

This effectively adds the behaviour of app.UseDefaultFiles() which will serve up /index.html upon request to /.

More info:
https://learn.microsoft.com/en-us/aspnet/core/fundamentals/static-files?view=aspnetcore-8.0#usefileserver-for-default-documents

[jwthomson]

This is roughly equivalent to Saturn's use_static which calls UseDefaultFile and UseStaticFiles - but a bit simpler.

For reference: https://github.com/SaturnFramework/Saturn/blob/4ae4d6126a6c7c0e91bded2ddbe1103030010c78/src/Saturn/Application.fs#L357