Fix xml syntax in org.freedesktop.portal.Flatpak.xml

A bit concerning that this does not break the build.
It did break the docs build in xdg-desktop-portal.

Closes: #2602
Approved by: mwleeds
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f