Update cheatsheet.fr.md

12.temporary_ins/90.electromagnetism-in-vacuum/20.electromagnetic-waves-vacuum/20.overview/cheatsheet.fr.md

@@ -191,6 +191,9 @@ figure à faire
     &\dfrac{\partial E_x}{\partial x}=\dfrac{\partial E_y}{\partial y}=0
     \end{align}\right\}`$
     **$`\large\boldsymbol{\mathbf{\Longrightarrow\;\begin{align}\\\dfrac{\partial E_z}{\partial z}=0\\(équa.1)\end{align}}}}`$**   
+    **$`\large\boldsymbol{\mathbf{\Longrightarrow\;\begin{align}\\
+        \large\boldsymbol{\mathbf{\dfrac{\partial E_z}{\partial z}=0\\(équa.1)}}}
+        \end{align}`$**   
     
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