ex-3: fix conceptual mistake

notes/sections/3.md

@@ -84,7 +84,8 @@ The transformation can be found by imposing the angular PDF to be a constant:
   \frac{d \theta}{dx} \right| \right. 
 \end{align*}
 
-Since $\theta \in [0, \pi]$, then the absolute value symbol can be omitted:
+If $\theta$ is chosen to grew together with $x$, then the absolute value can be
+omitted:
 
 \begin{align*}
   \frac{d \theta}{dx} = \frac{2}{\sin(\theta)}