Update cheatsheet.fr.md

12.temporary_ins/69.waves/30.n3/20.overview/cheatsheet.fr.md

@@ -1648,13 +1648,13 @@ $`\quad\quad \;\; = \cdots`$
 **$`\mathbf{U(\overrightarrow{r},t)}`$**   
 $`= 2\,A\,\,cos\big(\Delta \omega_{1-2} t + \varphi_B) \times cos\,(\omega_{moy} t + \varphi_A)`$   
 <br>
-**$`= \mathbf{
+**$`=\mathbf{
 \underbrace{
 2 A\,\,cos\left(2\pi\dfrac{\nu_1 - \nu_2}{2} t + \varphi_B\right)
 }_{
 \color{blue}{\text{terme lentement variable de fréquence} \\ \nu\, = \,\left|\frac{\nu_1 - \nu_2}{2}\right| = \left|\frac{440 - 441}{2}\right|=0,5\,Hz\,\lt\,20\,Hz \\ \text{terme de battement}
 }}}`$**   
-**\quad\quad\quad\mathbf{
+**$`\quad\quad\quad\mathbf{
 \times \underbrace{
 cos\,(\omega_{moy} t + \varphi_A)
 }_{