Remove possibly unused reference to "identity" in configuration from README

[wizhi]

The README mentions the need for a identity field in the configuration, but this seemingly isn't used anywhere.
I'd do a PR for it, but I'm uncertain whether this is intentional or not.

{"endpoint": "your-endpoint",
 "identity": "your-identity",
 "client_id": "your-client_id",
 "client_secret": "your-client-secret"}