Typo in attribute name

[psmiraglia]

According to saml-schema-metadata-2.0.xsd the attributes should be AuthnRequestsSigned and WantAssertionsSigned (missing s).

https://github.com/italia/spid-auth-docker/blob/a70ff5918ad665ac5712d08f10843f1387d417cc/opt/spid-metadata/transform.xsl#L24-L25