Update textbook.en.md

10.brainstorming-innovative-courses/intercambio-curso-electromagnetismo/textbook.en.md

@@ -227,7 +227,7 @@ EN : in orthonormal Cartesian coordinate :  <br>
 $`\Delta = \dfrac{\partial^2}{\partial x^2}+\dfrac{\partial^2}{\partial y^2}+\dfrac{\partial^2}{\partial z^2}`$
 
 $`\Delta\;\overrightarrow{U} = \overrightarrow{grad}\left( div\,\overrightarrow{U}\right) - \overrightarrow{rot}\left(\overrightarrow{rot}\,\overrightarrow{U}\right)`$   <br>
-$`\Delta\;\overrightarrow{U} = \overrightarrow{grad}\:div\,\overrightarrow{U} - \overrightarrow{rot}\:\overrightarrow{rot}\,\overrightarrow{U}`$   <br>
+$`\Delta\;\overrightarrow{U} = \overrightarrow{grad}\;div\;\overrightarrow{U} - \overrightarrow{rot}\;\overrightarrow{rot}\;\overrightarrow{U}`$   <br>
 ES : operador laplaciana vectorial, laplaciana vectorial, laplaciana de un campo vectorial   <br>
 FR : opérateur laplacien, laplacien, d'un champ scalaire ou d'un champ vecoriel   <br>
 EN : laplacian operator, vectorial laplacian, laplacian of a vector field <br>
@@ -235,7 +235,7 @@ in orthonormal Cartesian coordinate : <br>
 $`\Delta\;\overrightarrow{U} = \overrightarrow{e_x}\left(\dfrac{\partial^2\;U_x}{\partial x^2}+\dfrac{\partial^2\;U_x}{\partial y^2}+\dfrac{\partial^2\;U_x}{\partial z^2}\right)
 +\overrightarrow{e_y}\left(\dfrac{\partial^2\;U_y}{\partial x^2}+\dfrac{\partial^2\;U_y}{\partial y^2}+\dfrac{\partial^2\;U_y}{\partial z^2}\right)
 +\overrightarrow{e_z}\left(\dfrac{\partial^2\;U_z}{\partial x^2}+\dfrac{\partial^2\;U_z}{\partial y^2}+\dfrac{\partial^2\;U_z}{\partial z^2}\right)`$ <br>
-$`\Delta\;\overrightarrow{U} = \left \{
+$`\Delta\;\overrightarrow{U} = \left \|
    \begin{array}{r c l}
       \dfrac{\partial^2\;U_x}{\partial x^2}+\dfrac{\partial^2\;U_x}{\partial y^2}+\dfrac{\partial^2\;U_x}{\partial z^2} \\
       \dfrac{\partial^2\;U_y}{\partial x^2}+\dfrac{\partial^2\;U_y}{\partial y^2}+\dfrac{\partial^2\;U_y}{\partial z^2} \\