Update 13_semiconductors_solutions.md

src/13_semiconductors_solutions.md

@@ -13,7 +13,7 @@ Electrons near the top of the valence band have a negative effective mass, their
 
 ### Subquestion 2
 
-Holes near the top of the valence band have a positive effective mass, their energy decreases as $k$ increases from 0, and they have a negative group velocity for $k>0$.
+Holes near the top of the valence band have a positive effective mass, their energy increases as $k$ increases from 0, and they have a negative group velocity for $k>0$.
 
 ### Subquestion 3