Fix sign error in QED Lagrangian

script/chapter3.tex

@@ -820,7 +820,7 @@ \section{Gauge Theories}
 
 Gauge field $A_\mu$ is introduced into Lagrangian and a kinetic term is needed to obtain a wave equation is needed
 \begin{align}
-   \lag &= -\frac{1}{4}F^{\mu\nu}F_{\mu\nu} +  i \bar\psi \gamma^\mu \partial_\mu \psi - m \bar\psi \psi + e \bar\psi \gamma_\mu A^\mu \psi. \notag \\
+   \lag &= -\frac{1}{4}F^{\mu\nu}F_{\mu\nu} +  i \bar\psi \gamma^\mu \partial_\mu \psi - m \bar\psi \psi - e \bar\psi \gamma_\mu A^\mu \psi. \notag \\
 \end{align}
 Thus we have an additional Feynman rule
 \begin{align}