Update cheatsheet.en.md

01.curriculum/01.physics-chemistry-biology/02.Niv2/04.optics/04.use-of-basic-optical-elements/02.mirror/02.new-course-overview/cheatsheet.en.md

@@ -54,7 +54,7 @@ $`\Longrightarrow`$ A real object gives a virtual image.<br>
 
 * In each point of the spherical mirror, the law of reflection applies.
 * A spherical mirror is not stigmatic: The rays (or their extensions) coming from an object point generally do not converge towards an image point (see Fig. 2.)
-* A spherical mirrors with a limited aperture (see the angle $`\alpha`$ (rad) lower on Fig. 3. and 4.) and used so that 
+* A spherical mirrors with a limited aperture (see the angle $`\alpha`$ (rad) which is reduced on Fig. 3. and 4.) and used so that 
 angles of incidence remain small (see Fig. 4.) become quasi-stigmatic.
 
 ![](spherical-mirror-rays-stigmatism-1000-1.jpg)<br>