Update 3_drude_model.md - updated relevant chapter nrs and removed reference to exercises

src/3_drude_model.md

@@ -9,8 +9,7 @@ configure_plotting()
 ```
 
 # Drude theory and the free electron model
-_(based on chapters 3–4 of the book)_  
-Exercises 3.1, 3.3, 4.2, 4.5, 4.6, 4.7
+_(based on chapter 3 of the book)_  
 
 !!! summary "Learning goals"