fix typo in ac_power.md

uni/mmme/2051_electromechanical_devices/ac_power.md

@@ -27,8 +27,8 @@ $$P = \sqrt{3} V_lI_l\cos\gamma$$
 
   \begin{align*}
           v_{1N} = V\cos{2\pi ft} \\
-  v_{2N} = V\cos{2\pi ft - \frac{2\pi}{3} \\
-  v_{3N} = V\cos{2\pi ft + \frac{2\pi}{3}
+          v_{2N} = V\cos{2\pi ft - \frac{2\pi}{3}} \\
+          v_{3N} = V\cos{2\pi ft + \frac{2\pi}{3}}
   \end{align*}
 
 - Balanced loads and sources ensure that line/phase currents have equal magnitudes and that the