fix docs display (#507)

### What problem were solved in this pull request?

Issue Number: close #xxx

Problem:

### What is changed and how it works?

### Other information

docs/mkdocs.yml

@@ -63,10 +63,6 @@ nav:
   - 博客:
     - blog/introduction.md
     - blog/1.md
-  - 数据库系统实现原理与实践:
-    - db_course_lab/overview.md
-    - db_course_lab/cloudlab_setup.md
-    - db_course_lab/lab0.md
 
 markdown_extensions:
   - pymdownx.highlight: