Update cheatsheet.en.md

01.curriculum/01.physics-chemistry-biology/02.Niv2/04.optics/04.use-of-basic-optical-elements/01.plane-refracting-surface/02.plane-refracting-surface-overview/cheatsheet.en.md

@@ -80,7 +80,9 @@ refracting interface corresponds to two different plane refracting surfaces :<br
 #### Non stigmatism of spherical refracting surfaces
 
 Ray tracing study of a **spherical refracting surface** :
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 * **At each impact point** of the rays upon the spherical refracting surface, the **Snell-Descartes relation applies**.
 
@@ -222,8 +224,9 @@ in Figures 3 and 4, and 5 and 6 below, and review NOTE 1.
 !!!! also to check that you master the analytical study of each presented case.
 !!!!
 
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 * with **real objects**