Update link for SAML2 examples

On the page https://doc.nuxeo.com/nxdoc/saml-20-authentication/#integrating-the-nuxeo-platform-with-a-saml-20-idp there are examples linked to the nuxeo/nuxeo github repository: https://github.com/nuxeo/nuxeo/tree/master/nuxeo-services/login/nuxeo-platform-login-saml2/sample

With the reorganization on the master branch, the link has changed to:

https://github.com/nuxeo/nuxeo/tree/master/modules/platform/login/nuxeo-platform-login-saml2/sample

Previous versions should be updated as well.