Correct full namespace for mdui

[thijskh]

As listed in the OASIS standard

[c00kiemon5ter]

This change is correct.

[c00kiemon5ter]

Helpful for this is the metadata extensions spec: https://docs.oasis-open.org/security/saml/Post2.0/sstc-saml-metadata-ui/v1.0/os/sstc-saml-metadata-ui-v1.0-os.pdf

quoting the table from page 5:

Prefix	XML Namespace	Comments
md:	urn:oasis:names:tc:SAML:2.0:metadata	This is the SAML V2.0 metadata namespace defined in the SAML V2.0 metadata specification [SAML2Meta]
mdui:	urn:oasis:names:tc:SAML:metadata:ui	This is the SAML V2.0 metadata extension namespace defined by this document and its accompanying schema.
xsd:	http://www.w3.org/2001/XMLSchema	This namespace is defined in the W3C XML Schema specification [Schema1]. In schema listings, this is the default namespace and no prefix is shown.