Update cheatsheet.en.md

01.curriculum/01.physics-chemistry-biology/02.Niv2/04.optics/03.optical-systems-vergence/02.optical-systems-vergence-overview/cheatsheet.en.md

@@ -76,7 +76,7 @@ Fig. 2b. here the image points are virtual.
 
 #### Optical imager and basis physical principles.
 
-** Imager** :
+**Imager** :
 * **Intercept part of the light** emitted or diffused by the physical object.
 
 * *modify the pencils* of light from each object point, to give a **new pencil** that  :<br>