Update textbook.en.md

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

@@ -235,8 +235,9 @@ 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>
-$`\Delta\;\overrightarrow{U} = \left \|
-   \begin{array}{r c l}
+
+$`\Delta\;\overrightarrow{U} = \left |
+   \begin{array}
       \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} \\
       \dfrac{\partial^2\;U_z}{\partial x^2}+\dfrac{\partial^2\;U_z}{\partial y^2}+\dfrac{\partial^2\;U_z}{\partial z^2}