Update textbook.en.md

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

@@ -818,7 +818,7 @@ $`\displaystyle\oint_{\Gamma\,orient.}\overrightarrow{H} \cdot \overrightarrow{d
 \underset{S\,orient.}{\iint{\overrightarrow{j}\cdot\overrightarrow{dS}}}`$
 
 
-$`\left. \dfrac{dQ}{dt}\right|_S   =\oint_S \vec{j} \cdot \vec{dS}`$
+$`\displaystyle\left. \dfrac{dQ}{dt}\right|_S   =\oint_S \vec{j} \cdot \vec{dS}`$