Docs: Update link text for manifest (#10804)

CONTRIBUTING.md

@@ -244,7 +244,7 @@ If you'd like to contribute to the design or front-end, feel free to contribute
 
 Documentation is automatically synced from master to the [Gutenberg Documentation Website](https://wordpress.org/gutenberg/handbook/) every 15 minutes.
 
-To add a new documentation page, you'll have to create a Markdown file in the [docs](https://github.com/WordPress/gutenberg/tree/master/docs) folder and add an item to the [manifest file](https://github.com/WordPress/gutenberg/blob/master/docs/root-manifest.json).
+To add a new documentation page, you'll have to create a Markdown file in the [docs](https://github.com/WordPress/gutenberg/tree/master/docs) folder and add an item to the [root-manifest.json](https://github.com/WordPress/gutenberg/blob/master/docs/root-manifest.json).
 
 ### `@wordpress/component`