Update textbook.fr.md

00.brainstorming-pedagogical-teams/40.collection-existing-pedagogical-content/04.reference-frames-coordinate-systems/textbook.fr.md

@@ -57,6 +57,6 @@ $`dl=\sqrt{d\rho^2+ (\rho\,d\phi)^2+dz^2}`$
 
 $`M=M(\rho, \theta, \phi)`$
 
-$`dl=\sqrt{dr^2+(r\,d\theta)^2+(r\,sin\,\theta\,d\phi)^2}`$
+$`dl=\sqrt{dr^2+(r\,d\theta)^2+(r\,sin\theta\,d\phi)^2}`$