Add missing link to dev. certificate of origin.

Co-authored-by: Jyrki Gadinger <nilsding@nilsding.org>
Signed-off-by: Camila Ayres <hello@camilasan.com>

Author: camilasan

CONTRIBUTING.md

@@ -32,7 +32,7 @@ We use the Developer Certificate of Origin (DCO) as an additional safeguard
 for the Nextcloud project. This is a well established and widely used
 mechanism to assure contributors have confirmed their right to license
 their contribution under the project's license.
-Please read [contribute/developer-certificate-of-origin][dcofile].
+Please read [contribute/developer-certificate-of-origin](https://github.com/nextcloud/server/blob/master/contribute/developer-certificate-of-origin).
 If you can certify it, then just add a line to every git commit message:
 
 ```