Update 11_nearly_free_electron_model.md - updated relevant book chapter and lecture header

src/11_nearly_free_electron_model.md

@@ -6,8 +6,8 @@ import plotly.graph_objs as go
 pi = np.pi
 ```
 
-# Tight binding and nearly free electrons
-_(based on chapters 15–16 of the book)_  
+# The nearly free electron model & Bloch theorem
+_(based on chapter 15 of the book)_  
 
 !!! success "Expected prior knowledge"