Typo in excercises Einstein model

File in which the problem appears

1. 1_einstein_model.md
2. 1_einstein_model_solutions.md

Problematic sentence

1. Check that in the high temperature limit you get the same result as in Exercise 1.1.
2. In the high temperature limit \beta \rightarrow 0 and e^{\beta\hbar\omega} \approx 1 + \beta\hbar\omega, so C \rightarrow k_B which is the same result as in Exercise 1.1.

Correct version

1. Check that in the high temperature limit you get the same result as in Exercise 1.3.
2. In the high temperature limit \beta \rightarrow 0 and e^{\beta\hbar\omega} \approx 1 + \beta\hbar\omega, so C \rightarrow k_B which is the same result as in Exercise 1.3.