[docs] Fix typo and improve the wording a little

Created by: kopancek

Description

By fixing the typo we assure customers do not provide invalid configuration options in the SAML auth provider.

By clearing up the wording we make it explicit what we understand by group membership and that customers are expected to give us the data in a SAML Assertion Attribute.

Test plan

Eyballing 👀 . This is just a text change, I do not expect the docs to break as a result.