Update textbook.fr.md

00.brainstorming-pedagogical-teams/40.collection-existing-pedagogical-content/04.reference-frames-coordinate-systems/textbook.fr.md

@@ -339,9 +339,9 @@ Do we keep the same letter (S or A) to speak of both the surface and its area?
 Or do we separate the concepts? <br>
 If A is the area of a surface S, Which is the most logical and esay to understand for students
 among the following entries? <br>
-<br>$`dA=\iint_A dA`$ <br>
-$`dS=\iint_S dS`$ <br>
-$`dA=\iint_S dA`$ <br>
+<br>$`A=\iint_A dA`$ <br>
+$`S=\iint_S dS`$ <br>
+$`A=\iint_S dA`$ <br>
 
 [ES] Tenemos que elegir de forma independiente para cada idioma, entre estas notaciones :<br>
 [FR] Nous devons choisir de façon indépendante pour chaque langue, entre ces notations :<br>