docs: update outdated link

"Contributing" doc has moved from dev.chromium.org to Markdown file,
update the link.

Bug: None
Change-Id: I8574dec9fb980b347a024f067c0aa59637e6eaca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1678286Reviewed-by: Xiaochu Liu <xiaochu@chromium.org>
Commit-Queue: Xiaochu Liu <xiaochu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672658}

docs/linux_development.md

@@ -33,7 +33,7 @@ issues. If you can't find a related bug, please create a
 
 ## Contributing code
 
-See [Contributing code](https://dev.chromium.org/developers/contributing-code).
+See [Contributing code](contributing.md).
 
 ## Debugging