Fix typo. Exercise 3 had number 4.

e5ca00e8 · Joana Fraxanet Morales · c0a81acd · e5ca00e8
Commit e5ca00e8 authored 6 years ago by Joana Fraxanet Morales
--- a/src/1_einstein_model.md
+++ b/src/1_einstein_model.md
@@ -347,7 +347,7 @@ $$
  - What is the expectation value of $n$?


-### Exercise 4. Total heat capacity of a diatomic material
+### Exercise 3: Total heat capacity of a diatomic material

 Naturally occurring lithium has [two stable isotopes](https://en.wikipedia.org/wiki/Isotopes_of_lithium): $^6$Li  (7.5%) and $^7$Li (92.5%). Let us extend the Einstein model to take into account the different masses of different isotopes.