Update cheatsheet.en.md

01.curriculum/01.physics-chemistry-biology/03.Niv3/02.Geometrical-optics/02.geometrical-optics-foundings/01.concept-ray-of-light/02.concept-ray-of-light-overview/cheatsheet.en.md

@@ -53,7 +53,7 @@ I wanted to make this important remark here, but it is not simple: with Doppler
 !! $`n`$: complex value dependent on the $\nu$ frequency of the electromagnetic wave, strong variations representative of all light / matter interaction mechanisms: $`n (\nu) = \Re[n(\nu )] + \Im[n(\nu)]`$ <br>
 !!
 !! In the visible domain (where $`\lambda_0`$ is more used than $`\nu`$) and for transparent medium : <br>
-!! real value, small variations of $`n`$ with $`\lambda_0`$ ($`\frac{\Delta n}{n} <1 \%``$)
+!! real value, small variations of $`n`$ with $`\lambda_0`$ ($`\frac{\Delta n}{n} <1 \%`$)
 
 ##### Optical path