Update cheatsheet.fr.md

12.temporary_ins/40.classical-mechanics/20.n2/10.framework-of-classical-mechanics-2/20.overview/cheatsheet.fr.md

@@ -255,7 +255,7 @@ $`\color{green}{l_{AB}=\sqrt{(x_B-x_A)^2+(y_B-y_A)^2+(z_B-z_A)^2}}`$
 </span><span style="font-family:'Times New Roman';font-weight:bold;font-size:1.15rem;left;color:#bb3300"> durée  Δt<sub>1 2</sub></sub></span>
 <span style="font-family:'Times New Roman';font-weight:bold;font-size:0.85rem;color:#6699ff"> entre ces deux instants.</span>
 <span style="font-family:'Times New Roman';font-weight:bold;font-size:1.15rem;text-align:left;color:#bbbbbb">d'expression</span></p>
-$`\color{green}{|\Delta t_{1\,2}=t_2-t_1|=\sqrt{(t_2-t_1}^2}}`$</p>
+$`\color{green}{|\Delta t_{1\,2}=t_2-t_1|=\sqrt{(t_2-t_1)^2}}`$</p>
 <hr width=100% size=6px border-style="solid" color=#00bb33>