Update textbook.fr.md

00.brainstorming-pedagogical-teams/40.collection-existing-pedagogical-content/50.electromagnetism/40.n4/10.main/textbook.fr.md

@@ -353,8 +353,8 @@ $`fem = \mathcal{C}_E = \mathcal{E}
 [EN] (auto-trad) Ostrogradsky’s theorem = divergence theorem : for all vectorial field $`\vec{X}`$ :<br>
 
 [FR] (CME), [ES] (...)?, [EN] (...)? <br>
-$`\displaystyle\iiint_{\tau} div\;\overrightarrow{X} \cdot d\tau = \displaystyle 
-\oiint_{S\leftrightarrow\tau} \overrightarrow{X}\cdot\overrightarrow{dS}`$
+$`\displaystyle\iiint_{large\tau\normalsize} div\;\overrightarrow{X} \cdot d\tau = \displaystyle 
+\oiint_{S\leftrightarrow\large\tau\normalsize } \overrightarrow{X}\cdot\overrightarrow{dS}`$
 
 Stokes' theorem , for all vectorial field $`\vec{X}`$ :