Update docs/4_sommerfeld_model_solutions.md

docs/4_sommerfeld_model_solutions.md

@@ -88,7 +88,7 @@ We see that the free-electron density is much lower than the total electron dens
 
 ### Exercise 3: The electron dispersion, density of states, and heat capacity of graphene
 
-1. The electron dispersion is plotted here as a function of $k=|\mathbf{k}|=\sqrt{k_x^2+k_y^2}$
+1. The electron dispersion is plotted here as a function of $k_x$ for $k_y=0$.
 
 ```python
 import numpy as np