fix Katex errors

Operationsverstaerker/doc/Hinweise-OPV.md

@@ -245,7 +245,7 @@ $$
 \begin{split}
 &U_{e} = I\,R_{1},\qquad U_{a} = -I\,R_{2};\\
 &\\
-&U_{a} = -\frac{R_{2}}{R_{1}}\,U_{e}.
+&U_{a} = -\frac{R_{2}}{R_{1}}\,U_{e}.\\
 &\\
 &\\
 &v_{U} = \frac{U_{a}}{U_{e}} = -\frac{R_{2}}{R_{1}}