Update textbook.fr.md

12.temporary_ins/96.electromagnetism-in-media/20.reflexion-refraction-at-interfaces/20.metallic-waveguides/10.main/textbook.fr.md

@@ -26,7 +26,7 @@ _TE and TM waves and their corresponding standing wave behaviour along the y axi
 
 _equations_
 
-$`\overrightarrow{E}_{\perp}=\underset{-2\,E_0\,\sin(k\,y\,cos\,\theta)}{amplitude}`$
+$`\overrightarrow{E}_{\perp}=\underbrace{-2\,E_0\,\sin(k\,y\,cos\,\theta)}_{amplitude}`$
 
 <!------------
 []{#_bookmark79 .anchor}$`\overrightarrow{E}`$~⊥~ = −2*E*~0~ sin (*k y* cos *θ*) sin