Update mkdocs.yml

mkdocs.yml

@@ -46,7 +46,7 @@ nav:
     - 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'
+    - Band structures in 2D: '12_band_structures_in_higher_dimensions_solutions.md'
     # - Basic principles: '13_semiconductors_solutions.md'
     # - Doping and devices: '14_doping_and_devices_solutions.md'