Update the RP-initiated Logout links

[rozagerardo]

Describe the bug
We're using the following link when we refer to the OIDC RP-Initiated Logout feature:

https://openid.net/specs/openid-connect-session-1_0.html#RPLogout

But it seems this spec is not related to the feature anymore, in fact, the #RPLogout fragment doesn't seem to correspond to any section/element in the document.

This seems to be the correct link to the RP-Initiated spec:

https://openid.net/specs/openid-connect-rpinitiated-1_0.html

To Reproduce
Browse the following section in our docs:

https://docs.spring.io/spring-security/reference/servlet/oauth2/login/advanced.html#oauth2login-advanced-oidc-logout

and click on the following link:

One of the strategies available is RP-Initiated Logout.

The link will redirect to an OIDC spec that doesn't contain much RP-Initiated Logout info, which might be a little misleading.

Expected behavior
The links should point to the correct OIDC Spec:

https://openid.net/specs/openid-connect-rpinitiated-1_0.html

Sample
N/A

[rozagerardo]

Here is a small PR to fix this, in case you agree the links should be updated:

#12082