docs(v2): minor typo fix (#3423)

* docs: minor typo fix 

Markdown import example in website/src/pages/examples/markdownPageExample.md imports the wrong chapter.
See this comment f234c40#r42067403

### Have you read the [ Contributing Guidelines on pull requests](https://github.com/facebook/docusaurus/blob/master/CONTRIBUTING.md#pull-requests)?
(Yes)

* Update website/src/pages/examples/markdownPageExample.md

Co-authored-by: Sébastien Lorber <slorber@users.noreply.github.com>

website/src/pages/examples/markdownPageExample.md

@@ -53,7 +53,7 @@ import Chapter2 from './_chapter2.mdx';
 <Chapter2 />;
 ```
 
-import Chapter1 from './\_chapter2.mdx';
+import Chapter1 from './\_chapter1.md';
 
 <Chapter1/>