Update contributing.md

Co-authored-by: Balázs Orbán <info@balazsorban.com>

contributing.md

@@ -198,7 +198,7 @@ Next.js version at the same time (some changes might require re-running `yarn ne
 
 ## Updating documentation paths
 
-Our documentation currently leverages a [manifest file](/docs/manifest.json) which is what how documentation entries are checked.
+Our documentation currently leverages a [manifest file](/docs/manifest.json) which is how documentation entries are checked.
 
 When adding a new entry under an existing category you only need to add an entry with `{title: '', path: '/docs/path/to/file.md'}`. The "title" is what is shown on the sidebar.