fix missing raw string

src/4_sommerfeld_model_solutions.md

@@ -135,8 +135,8 @@ dEmin = integrate.quad(integral, 0, 1e1, args=(T-dT))[0] - 0.8e10 * 5.2**(5./2)
 
 CV = (dEplus - dEmin) / (2*dT);
 
-print('dE = {:.4e} eV'.format(dE))
-print('Cv = {:.4e} eV/K'.format(CV))
+print(f'dE = {dE:.4e} eV')
+print(f'Cv = {CV:.4e} eV/K')
 ```
 Check the source code written in python for solving integral using midpoint rule.
 
@@ -163,7 +163,7 @@ ax.set_xticks([])
 ax.set_yticks([])
 
 ax.set_xlabel(r'$\mid \vec k \mid$', fontsize=14)
-ax.set_ylabel('$\varepsilon$', fontsize=18, rotation='horizontal')
+ax.set_ylabel(r'$\varepsilon$', fontsize=18, rotation='horizontal')
 ax.yaxis.set_label_coords(0.5,1)
 ax.xaxis.set_label_coords(1.0, 0.49)
 ```