Minor equation fix

src/1_complex_numbers.md

@@ -124,8 +124,8 @@ $$\varphi = \begin{cases} \arctan(b/a) &{\rm for ~} a>0; \\
 When increasing $\varphi$ with $2 \pi$, we make a full circle and reach the same point on the complex plane. In other words, when adding $2 \pi$ to our argument, we get the same complex number!
 As a result, the argument $\varphi$ is defined up to $2 \pi$, and we are free to make any choice we like:
 $$\begin{align}
--\pi < \varphi < \pi $ {\rm (left image)} \\
--\frac{\pi}{2} < \varphi < \frac{3 \pi}{2} $ {\rm (right image)} \end{align} $$
+-\pi < \varphi < \pi  {\rm (left image)} \\
+-\frac{\pi}{2} < \varphi < \frac{3 \pi}{2} {\rm (right image)} \end{align} $$
 
 ![image](figures/complex_numbers_11_0.svg)