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

@@ -192,7 +192,7 @@ $`\Longrightarrow\overline{SF}=-\dfrac{n_{inc}\cdot\overline{SC}}{n_{eme}-n_{inc
 ##### 2 - Thin spherical refracting surface representation
 
 * **Optical axis = revolution axis** of the refracting surface, positively **oriented** in the direction of 
-propagation of the light (from the object towards the refracting surface)
+propagation of the light.
 
 * Thin spherical refracting surface representation :<br><br>
 \- **line segment**, perpendicular to the optical axis, centered on the axis with symbolic