show lec 9 solutions

mkdocs.yml

@@ -43,7 +43,7 @@ nav:
     - Bonds and spectra: '6_bonds_and_spectra_solutions.md'
     - Tight-binding model: '7_tight_binding_model_solutions.md'
     - Many atoms per unit cell: '8_many_atoms_solutions.md'
-    # - Crystal structure: '9_crystal_structure_solutions.md'
+    - Crystal structure: '9_crystal_structure_solutions.md'
     # - X-ray diffraction: '10_xray_solutions.md'
     # - Nearly free electron model: '11_nearly_free_electron_model_solutions.md'
     # - Band structures in 2D: '12_band_structures_in_higher_dimensions_solutions.md'