Update textbook.fr.md

01.curriculum/01.physics-chemistry-biology/04.Niv4/04.electromagnetism/02.electromagnetic-waves-vacuum/02.electromagnetic-waves-vacuum-main/textbook.fr.md

@@ -220,7 +220,9 @@ Jean & 12,3 & Admis\\
 \overrightarrow{B}(\overrightarrow{r},t)=\overrightarrow{B}(\overrightarrow{u}\cdot\overrightarrow{r}-ct)\\
 \end{eqnarray}-->
 
-$`\left \{
+Onde EM plane progressive :<br>
+$`\Longleftrightarrow 
+\left \{
    \begin{array}{r c l}
       AB  & = & 192 \\
       C   & = & 5\,896 \\