[po/fr] Correct the translation for "Lock Screen"

In the background capplet, the string for "lock screen" was translated
"Verrouiller l'écran"
(https://git.gnome.org/browse/gnome-control-center/tree/po/fr.po#n59) but lock
is not a verb in this case but a noon.
So the translation should rather be something like "Écran de verrouillage"
Comment added accordingly to builder file.

https://bugzilla.gnome.org/show_bug.cgi?id=723967