Fixed "SecureUser" references in documentation

I've fixed a couple of inconsistencies between the documentation in the README and the implementation of `SecureUser`.

Previously: the docs referred to `SecureUserFunc`, which does not exist. I've changed the docs to properly use `SecureUser` in the example when passing additional user details.

I've also updated the Google example to include the additional user details as a second route under `/private/user` and corrected a discrepancy between the Google example docs and the actual flag expected in the example (now properly refers to `access_key`).

The examples run and work as expected. I've attempted to copy the existing approach/templates where possible to maintain a consistent style.