Fix nav (foreword -> preface)

Author: taoky

mkdocs.yml

@@ -91,7 +91,7 @@ plugins:
 
 nav:
   - 首页: index.md
-  - 前言: foreword.md
+  - 前言: preface.md
   - 章节编写指导: spec/writing.md
   - 贡献者: spec/contributors.md
   - 运维基础: