Fix trailing slash

Getting the following when verifying JWT in IDhub (possibly first service doing this?)

INFO:apigatewayauth.authentication:Incoming API token failed verification: Token has wrong audience https://webapp-{foobar}-nw.a.run.app, expected one of ['https://webapp-{foobar}-nw.a.run.app/']

Could override expected audience via env var but better the default behaves the same way as the API Gateway creates the JWT.

Part of https://gitlab.developers.cam.ac.uk/uis/devops/iam/idhub/api/-/issues/90