Update _zh-cn/overviews/scala3-book/domain-modeling-oop.md

Co-authored-by: 梦境迷离 <[email protected]>

Author: benluo

_zh-cn/overviews/scala3-book/domain-modeling-oop.md

@@ -131,7 +131,7 @@ val s4: Showable = s1
 // ... and so on ...
 ```
 
-#### 扩展计划
+#### 扩展规划
 
 如前所述，可以扩展另一个类：