doc: update link to code-of-conduct.md (#49)

Refs: nodejs/node#41408

The code-of-conduct.md file has been moved, update
the related links.

Signed-off-by: Michael Dawson <mdawson@devrus.com>

CONTRIBUTING.md

@@ -6,7 +6,7 @@ The Node.js project has a
 [Code of Conduct](https://github.com/nodejs/admin/blob/master/CODE_OF_CONDUCT.md)
 to which all contributors must adhere.
 
-See [details on our policy on Code of Conduct](https://github.com/nodejs/node/blob/master/doc/guides/contributing/code-of-conduct.md).
+See [details on our policy on Code of Conduct](https://github.com/nodejs/node/blob/master/doc/contributing/code-of-conduct.md).
 
 <a id="developers-certificate-of-origin"></a>
 ## Developer's Certificate of Origin 1.1