Update textbook.fr.md

00.brainstorming-pedagogical-teams/40.collection-existing-pedagogical-content/05.classical-mechanics/vector-analysis/textbook.fr.md

@@ -308,7 +308,7 @@ $`\;\Longrightarrow\left|\overrightarrow{U}\cdot\overrightarrow{V}=+||\overright
  \overrightarrow{U}\cdot\overrightarrow{V}=-||\overrightarrow{U}||\cdot||\overrightarrow{V}||
 \right.`$ 
 
-$` \underline{\overrightarrow{B}}^{\,0}_{\,ref}=\left|\begin{array}{l} +\,B_{ref}^0\,cos\,\theta_{ref} \\ 0 
+$` \underline{\overrightarrow{B}}^{\,0}_{\,ref}=\left|\begin{array}{l} +\,B_{ref}^0\,cos\,\theta_{ref} 
 \\ +\,B_{ref}^0\,sin\,\theta_{ref} \end{array}\right.\quad , \quad `$ 
 
 $` \underline{\overrightarrow{B}}^{\,0}_{\,trans}=\left|\begin{array}{l} -\,B_{trans}^0\,cos\,\theta_{trans} \\ 0