Update callback route to remove `/implicit`

[bdemers]

The okta-hosted example (and I'm guessing the others too) don't use the implicit flow 🎉 but the callback URL is still /implicit/callback)

This example should be updated to use just /callback instead

[shuowu]

Thanks @bdemers Internal Ref: OKTA-324376

[swiftone]

Completed - though we switched to /login/callback