Update cheatsheet.en.md

12.temporary_ins/10.electrostatics-vacuum/30.gauss-theorem-demonstration/20.overview/cheatsheet.en.md

@@ -1,3 +1,13 @@
+---
+title: Demonstration of the Gauss's theorem
+published: true
+routable: false
+visible: false
+lessons:
+    - slug: gauss-ampere-theorems-demonstration
+      order: 2
+---
+
 <!--caligraphie de l'intégrale double curviligne-->
 $`\def\oiint{\displaystyle\mathop{{\iint}\mkern-18mu \scriptsize \bigcirc}}`$
 $`\def\Ltau{\Large{\tau}\normalsize}`$