Update example to simplify parameters
Our apache-saml2 example uses ${IDP_URL} for the Shibboleth IDP. This gets set, non-obviously, in the Dockerfile. Update the xml file to hard-code the value since we shouldn't have complexity in the example for no good reason.
Closes https://gitlab.developers.cam.ac.uk/uis/devops/raven/docs/-/issues/6
Edited by Dr Abraham Martin