use proper headings for titles

notes/sections/4.md

@@ -1,6 +1,6 @@
 # Exercise 4
 
-**Kinematic dip pdf derivation**
+## Kinematic dip PDF derivation
 
 Consider a great number of non-interacting particles, each of which with a
 random momentum $\vec{p}$ with module between 0 and $p_{\text{max}}$ randomly
@@ -214,7 +214,8 @@ Namely:
 }
 \end{figure}
 
-**The code**
+
+## Monte Carlo simulation
 
 The same distribution should be found by generating and binning points in a
 proper way.