fix(example): incorrect callback urls after stripping route prefix

[persidskiy]

Fixes the wrong redirect url in the example.

Step to reproduce:

1. set up google provider in _example/main.go, run the app
2. open http://localhost:8080/web, click google
3. google oauth handler errors with Error 400: redirect_uri_mismatch

This happens due to r.URL.Path being /google/login instead of /auth/google/login in LoginHander https://github.com/go-pkgz/auth/blob/master/v2/provider/oauth2.go#L143 .