Update mkdocs.yml

Author: HowCam

mkdocs.yml

@@ -111,6 +111,7 @@ nav:
     - 高中部分:
       - U1 走近细胞: biology/senior/U1.md
       - 生物必修一: biology/senior/BX-1.md
+      - 生物选择性必修三: biology/senior/XB-3.md
 
 theme:
   name: material