Fix README.md
Fix README.md

Remove keyword argument from "Saml::Bindings::HTTPPost.receive_message" method in "Using libsaml as an IDP" section. The second argument for Saml::Bindings::HTTPPost.receive_message does not have a 'type:' keyword argument (https://github.com/digidentity/libsaml/blob/master/lib/saml/bindings/http_post.rb#L22)
digidentity/libsaml,JanDintel/libsaml