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

Update 12.temporary_ins/90.electromagnetism-in-vacuum/10.maxwell-equations/20.overview/cheatsheet.fr.md

Deleted 12.temporary_ins/10.electrostatics-vacuum/40.gauss-theorem-applications/05.gauss-symetries-invariances/20.cheatsheet.fr.md/.gitkeep