Update textbook.fr.md

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

@@ -268,7 +268,7 @@ The power density (power per unit surface, units $`[W:m^2]`$)
 traversing the waveguide can be evaluated from the time-averaged
 Poynting vector :
 
-$`\langle\overrightarrow{P}=\left\langle\dfrac{\overrightarrow{E}\land\overrightarrow{B}}{\mic}\right\rangle`$
+$`\langle\overrightarrow{P}=\left\langle\dfrac{\overrightarrow{E}\times\overrightarrow{B}}{\micron}\right\rangle`$