Update commit command to use '-s' flag for signing

Signed-off-by: Jan Kowalleck <jan.kowalleck@gmail.com>

Author: jkowalleck

CONTRIBUTING.md

@@ -49,5 +49,5 @@ Please sign off your commits, to show that you agree to publish your changes und
 , and to indicate agreement with [Developer Certificate of Origin (DCO)](https://developercertificate.org/).
 
 ```shell
-git commit --signed-off ...
+git commit -s ...
 ```