docs(Collapse): fix invalid description of demo `extra` (#42521)

* fix: collapse doc

* fix collapse extra demo description

* Update components/collapse/demo/extra.md

Co-authored-by: afc163 <afc163@gmail.com>

---------

Co-authored-by: afc163 <afc163@gmail.com>

components/collapse/demo/extra.md

@@ -1,7 +1,7 @@
 ## zh-CN
 
-可以同时展开多个面板，这个例子默认展开了第一个。
+自定义渲染每个面板右上角的内容。
 
 ## en-US
 
-More than one panel can be expanded at a time, the first panel is initialized to be active in this case.
+Render extra element in the top-right corner of each panel.