Update solution nearly free electron model exercise 2.3

docs/11_nearly_free_electron_model_solutions.md

@@ -91,7 +91,7 @@ def dispersions(N = 5):
     plt.xlabel('$k$',fontsize=19)
     plt.ylabel('$E$',fontsize=19)
     plt.xticks((-2*pi, -pi, 0 , pi,2*pi),('$-2\pi/a$','$-\pi/a$','$0$','$\pi/a$','$2\pi/a$'),fontsize=15)
-    plt.yticks((1,27,54),('$E_0$','$E_1$','$E_{-1}$'))
+    plt.yticks((1,27,54),('$E_0$','$E_{-1}$','$E_{1}$'))
     
 dispersions(5)
 ```