doc: update IANA MIME types registry URL

Use a more official one, served over HTTP rather than FTP.

Reviewed-by: Matheus Santana <embs@cin.ufpe.br>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
1 job for master in 1 minute and 46 seconds
Status Job ID Name Coverage
  Build
passed #6277
build-native

00:01:46