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

@@ -66,7 +66,7 @@ I wanted to make this important remark here, but it is not simple: with Doppler
 * **$`\mathrm{d}\delta_P`$** : *infinitesimal optical path at point P on path $`\Gamma`$*
 
 Optical path along a path between 2 fixed points A and B :
-**$`\delta\;=\;\displaystyle\int_{P \in \Gamma}\mathrm{d}\delta_P\;`$$`=\;\int_{P \in \Gamma}n_P\cdot\mathrm{d}s_P`$**
+**$`\delta\;=\;\displaystyle\int_{P \in \Gamma}\mathrm{d}\delta_P\;`$$`=\;\displaystyle\int_{P \in \Gamma}n_P\cdot\mathrm{d}s_P`$**
 
 * **$`\delta`$** $`=\displaystyle\int_{\Gamma}n\cdot\mathrm{d}s\;=\;\int_{\Gamma}\dfrac{c}{v}\cdot\mathrm{d}s`$ = $`c\;\displaystyle\int_{\Gamma}\dfrac{\mathrm{d}s}{v}`$ = *$`\;c\;\tau`$*
 * **$`\delta`$** is *proportional to the travel time*.