Fix typo

Author: wooorm

readme.md

@@ -967,7 +967,7 @@ Yields:
 ## Contribute
 
 **mdast** is built by people just like you!  Check out
-[`contribute.md`][contribute] for ways to get started.
+[`contributing.md`][contributing] for ways to get started.
 
 This project has a [Code of Conduct][coc].  By interacting with this repository,
 organisation, or community you agree to abide by its terms.
@@ -1055,7 +1055,7 @@ Thanks to
 
 [frontmatter]: https://github.com/wooorm/remark-frontmatter
 
-[contribute]: contributing.md
+[contributing]: contributing.md
 
 [coc]: code-of-conduct.md