Minor revisions of flatpak(1)
Minor revisions of flatpak(1)

Capitalize Flatpak in text, add a paragraph about building
that references flatpak-builder, and add a reference to
www.flatpak.org.

Closes: #2410
Approved by: alexlarsson

matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak