fix lect 9 sol

src/9_crystal_structure_solutions.md

@@ -8,7 +8,7 @@ from math import pi
 ## Exercise 1: Diatomic crystal¶
 
 ### Subquestion 1
-```
+```python
 y = np.repeat(np.arange(0,8,2),4)
 x = np.tile(np.arange(0,8,2),4)
 plt.figure(figsize=(5,5))