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

@@ -83,7 +83,7 @@ Fig. 2b. here the image points are virtual.
 \- *converges* into a new point in space $\Longrightarrow$ *concentration of light energy* at this point.
 \- *diverges* from on other new point in space $\Longrightarrow$ *no concentration of light energy* at this point.
 
-** Optical imager** :
+**Optical imager** :
 * create a **true three dimensional** *image* of the extended object surface oriented towards the imager.
 * use **refraction and/or reflection** phenomena.
 <!-- In main or beyond, talk about this, in a comment. Term "optical", contradiction. "visible?" or "refraction/reflexion use?". Holographic image work in the visible range too, but is based on diffraction pehnomenon.