Update documentation on ID token signature verification.

[chamathns]

Description:
Currently, in the documentation [1], the approach mentioned for ID token signature verification involves extracting the public key from the wso2carbon.jks and then, verifying the signed ID token.
However, the recommended approach should be to get the JWKeySet from the /jwks endpoint and verifying the signed ID token with that.

[1] - https://is.docs.wso2.com/en/latest/learn/idtoken-signature-verification/