Pontoon: Update French (fr) localization of Firefox Monitor Website

Localization authors:
- Théo Chevalier <theo.chevalier11@gmail.com>

locales/fr/app.ftl

@@ -15,6 +15,7 @@
 -brand-pocket = Pocket
 -brand-lockwise = Firefox Lockwise
 -brand-send = Firefox Send
+-brand-fpn = Firefox Private Network