Update annex.fr.md

00.brainstorming-pedagogical-teams/45.synthesis-structuring/instructions-for-levels/30.beyond/annex.fr.md

@@ -292,7 +292,7 @@ Sont proposées les catégories suivantes, mais à débattre, toutes les idées
 ! A lensball is a polished spherical ball of radius $`R=5 cm`$, made of glass of refractive 
 ! index value $`n_{glass}=1.5`$. The cathedral is 90 meters high, and you stand with the lensball 
 ! 400 meters from the cathedral. You look at the cathedral through the lens, your eye being at
-! 20 cm from the center of the lens. What would you expect to see? 
+! 20 cm from the lens. What would you expect to see? 
 !
 ! * _The resolution time is the typical expected time to be allocated to this problem_
 ! _if it was part of an examen for an optics certificate_.