Update saml.md, specifying that issuer should be instance domain name.

What does this MR do?

Specify the value required for the issuer field in SAML configuration

Are there points in the code the reviewer needs to double check?

NO

Why was this MR needed?

A client configured specified a url given by OneLogin as issuer, instead of the instance's domain name. I believe it will be clearer in the docs if it specifies that the instance domain name should be used as the issuer

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

nill

What are the relevant issue numbers?

None