doc/flatpak-config: Fix example command
doc/flatpak-config: Fix example command

The key is "languages" not "language", and the value needs to be quoted.

Closes: #2361
Approved by: matthiasclasen

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