Update textbook.fr.md

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

@@ -262,7 +262,7 @@ $`+(2\,dl_x\,dl_y)\,(\overrightarrow{e_x}\cdot\overrightarrow{e_y})`$
 $`+(2\,dl_x\,dl_z)\,(\overrightarrow{e_x}\cdot\overrightarrow{e_z})`$
 $`+(2\,dl_y\,dl_z)\,(\overrightarrow{e_y}\cdot\overrightarrow{e_z})`$
 $`=\sqrt{(dl_x)^2+(dl_y)^2+(dl_z)^2}`$
-$`=\sqrt{dx^2+dy^2+dz^2}`$
+$`=\sqrt{dx^2+dy^2+dz^2}=dl`$
 
 
 * **N3 ($`\rightarrow`$ N4)**<br>