Update textbook.en.md

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

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