typo sommerfeld solutions

docs/4_sommerfeld_model_solutions.md

@@ -108,7 +108,7 @@ ax.spines['top'].set_color('none')
 ax.set_xticks([])
 ax.set_yticks([])
 
-ax.set_xlabel(r'$k$', fontsize=14)
+ax.set_xlabel(r'$k_x$', fontsize=14)
 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)