Update cheatsheet.fr.md

12.temporary_ins/08.grad-div-rot/20.div/20.overview/cheatsheet.fr.md

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 ##### Expression de la divergence en coordonnées cylindriques
 
-**$`\mathbf{div\,\overrightarrow{X}}`$**$`\boldsymbol{\mathbf{\;=\dfrac{d\Phi_X}{d\tau}}}`$**$`\boldsymbol{\mathbf{\;=\dfrac{1}{\rho}\;\dfrac{\partial\,(\rho\,X_{\rho})}{\partial\,\rho}
+**$`\mathbf{div\,\overrightarrow{X}}`$**$`\boldsymbol{\mathbf{\;=\dfrac{d\Phi_X}{d\tau}}}`$**$`\boldsymbol{\mathbf{\;=\dfrac{1}{\rho}\;\dfrac{\partial\,(\,\rho\,X_{\rho})}{\partial\,\rho}
 +\dfrac{1}{\rho}\;\dfrac{\partial\,X_{\varphi}}{\partial\,\varphi}+\dfrac{\partial\,X_{z}}{\partial\,z}}}`$**
 
 <br>**$`\mathbf{