Update textbook.fr.md

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

@@ -244,7 +244,7 @@ $`dx`$, $`dy`$ y $`dz`$, et il s'écrit :<br>
 in cartesian coordinates is the displacement vector from point $`M(x,y,z)`$ to point 
 $`M'(x+dx,y+dy,z+dz)`$ when the coordinates vary infinitely in quantities $`dx`$, $`dy`$ y $`dz`$,
 and it writes :<br>
-<br>$`=\overrightarrow{MM'}=d\overrightarrow{OM}=\overrightarrow{dr}=\overrightarrow{dl}`$
+<br>$`\overrightarrow{MM'}=d\overrightarrow{OM}=\overrightarrow{dr}=\overrightarrow{dl}`$
 $`=\partial\overrightarrow{OM}_x+\partial\overrightarrow{OM}_y+\partial\overrightarrow{OM}_z`$
 $`=\overrightarrow{dl_x}+\overrightarrow{dl_y}+\overrightarrow{dl_z}`$
 $`=l_x\;\overrightarrow{e_x}+l_y\;\overrightarrow{e_y}+l_z\;\overrightarrow{e_z}`$