Fix URL and reference style for IRT document

[nigelmegitt]

The IRT Technical Guidelines referenced for the ede1 entry currently has two issues:

1. The reference style is wrong - it is a direct href instead of a link via the Normative References section
2. The link gives a 404.

@TairT has told me out of band that the document has moved to https://www.irt.de/fileadmin/media/Neue_Downloads/Publikationen/Technische_Richtlinien/EBU-TT-D-Basic-DE-Subtitle_Format_ARD_Mediathek_Portals-v1.2.pdf