Update textbook.en.md

10.brainstorming-innovative-courses/intercambio-curso-electromagnetismo/textbook.en.md

@@ -762,7 +762,7 @@ Ostrogradsky’s theorem = divergence theorem (= Gauss's theorem) :
 
 for all vectorial field $`\vec{X}`$, 
 
-$`\displaystyle\iiint_{\tau} div\;\overrightarrow{X} \cdot d\tau= \displaystyle\oiint_{S\leftrightarrow\tau} \overrightarrow{X}}\cdot\overrightarrow{dS}`$
+$`\displaystyle\iiint_{\tau} div\;\overrightarrow{X} \cdot d\tau = \displaystyle \oiint_{S\leftrightarrow\tau} \overrightarrow{X}\cdot\overrightarrow{dS}`$