Doc: fix broken link

Author: zhxt

CONTRIBUTING.md

@@ -46,7 +46,7 @@ You can use command `bash apollo.sh test` to run all unit tests.
 
 If your code is not straightforward for other contributors to understand, it is recommended to implement the code in a clear and efficient way, and provide sufficient documentation.
 Apollo uses doxygen to help generate formatted API Document with command `bash apollo.sh doc`.
-Document your code following this guide [How to document code](how_to_document_code.md).
+Document your code following this guide [How to document code](docs/howto/how_to_document_code.md).
 
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