Fix incorrect OpenIDConnect discovery url

[shilpa-padgaonkar]

Fixes #3152

[shilpa-padgaonkar]

@handrews : As discussed in the last call, I have made the changes to the existing PR now.

Just to elaborate the changes again:

The original spec had this text

OAuth2's common flows (implicit, password, client credentials and authorization code) as defined in RFC6749, and OpenID Connect Discovery

The link to OpenID Connect discovery was mistakenly pointed to Oauth2 discovery specs - https://tools.ietf.org/html/draft-ietf-oauth-discovery-06.
I corrected this in my original PR to point it to OpenID Connect Discovery specs.

I later realized that the text (or reference) itself doesn't fit here, and it should rather be OpenID Connect Core (and not OpenID Connect Discovery) which would then refer to the link - https://openid.net/specs/openid-connect-core-1_0.html.

I have now made both these changes to the existing PR. Could you kindly check?

[handrews]

@lornajane were you the one that asked for changes here? All I remember that is that it quickly exceeded my knowledge in this area. @shilpa-padgaonkar's comment makes sense to me so my approval still stands.