Update 4_sommerfeld_model.md - added reference to chapter of book

src/4_sommerfeld_model.md

@@ -7,8 +7,7 @@ from common import draw_classic_axes, configure_plotting
 
 configure_plotting()
 ```
-
-### Sommerfeld theory (free electron model)
+_(based on chapter 4 of the book)_  
 
 !!! summary "Learning goals"
 
@@ -20,6 +19,7 @@ configure_plotting()
     - calculate the electron contribution to the specific heat of a solid.
     - describe central terms such as the Fermi energy, Fermi temperature, and Fermi wavevector.
 
+### Sommerfeld theory (free electron model)
 
 Atoms in a metal provide conduction electrons from their outer shells (often s-shells). These can be described as waves in the crystal, analogous to phonons. Hamiltonian of a free electron: