Update cheatsheet.fr.md

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

@@ -1647,7 +1647,7 @@ $`\quad\quad \;\; = \cdots`$
    <br>
    $`\color{brown}{\mathbf{U(\overrightarrow{r},t)}} = 2\,A\,\,cos\big(\Delta \omega_{1-2} t + \varphi_B) \times cos\,(\omega_{moy} t + \varphi_A)`$   
 
-   $`\quad = \mathbf{\underbrace{2\,A\,\,cos\left(2\pi\dfrac{\nu_1 - \nu_2}{2} t + \varphi_B}_{
+   $`\quad = \mathbf{\underbrace{2\,A\,\,cos\left(2\pi\dfrac{\nu_1 - \nu_2}{2} t + \varphi_B\right)}_{
    \color{blue}{\text{terme de fréquence} \\ \left|\dfrac{\nu_1 - \nu_2}{2}\right| = \left|\dfrac{440 - 441}{2}\right|=0,5\,Hz\;\lt\;10\,Hz}}
    \right) \times cos\,(\omega_{moy} t + \varphi_A)}`$