be more specific in a learning goal

src/4_sommerfeld_model.md

@@ -15,7 +15,7 @@ configure_plotting()
     After this lecture you will be able to:
 
     - calculate the electron density of states in 1D, 2D, and 3D using the Sommerfeld free-electron model.
-    - express the number and energy of particles in a system in terms of integrals over k-space.
+    - express the number and energy of electrons in a system in terms of integrals over k-space.
     - use the Fermi distribution to extend the previous learning goal to finite T.
     - calculate the electron contribution to the specific heat of a solid.
     - describe central terms such as the Fermi energy, Fermi temperature, and Fermi wavevector.