Meta: correct typo in README (#6)

Author: annevk

README.md

@@ -18,7 +18,7 @@ started, leave a comment on the issue or ask around [on chat](https://whatwg.org
 
 ## Pull requests
 
-In short, change `undex.bs` and submit your patch, with a
+In short, change `index.bs` and submit your patch, with a
 [good commit message](https://github.com/whatwg/meta/blob/main/COMMITTING.md). Consider
 reading through the [WHATWG FAQ](https://whatwg.org/faq) if you are new here.