typo fix

src/11_nearly_free_electron_model_solutions.md

@@ -110,7 +110,7 @@ V_{m-n} & \dfrac{\hbar^2 k_m^2}{2m}
 \end{equation}
 The eigenvalues of this fellow are 
 \begin{equation}
-E=\frac{\hbar^2(k_n^2+k_m^2)}{4m}\pm\sqrt{\frac{\hbar^4}{16m^2}(k_n^2-k_m^2)+|V_{n-m}|^2}.
+E=\frac{\hbar^2(k_n^2+k_m^2)}{4m}\pm\sqrt{\frac{\hbar^4}{16m^2}(k_n^2-k_m^2)^2+|V_{n-m}|^2}.
 \end{equation}
 This clearly displays that only if $|k_n|\approx |k_m|$, the band structure will be affected (given that the potential is weak, and therefore small). This nicely demonstrates how an avoided crossing arises.