tokio: fix broken contributing guide link (tokio-rs#2267)

The link to the contributing guide in the tokio sub crate was
referencing a non-existent file. This updates the link to reference
the repo root's CONTRIBUTING.md file.

Fixes: tokio-rs#2266

README.md

@@ -110,7 +110,7 @@ question. Last, if that doesn't work, try opening an [issue] with the question.
 you! We have a [contributing guide][guide] to help you get involved in the Tokio
 project.
 
-[guide]: CONTRIBUTING.md
+[guide]: https://github.com/tokio-rs/tokio/blob/master/CONTRIBUTING.md
 
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