contributing: Fix negation grammar mistake in DCO chapter

This change is trivial it should be easy to merge.

Relate-to: https://git.ostc-eu.org/OSTC/OHOS/docs/-/issues/74
Forwarded: https://git.ostc-eu.org/OSTC/OHOS/docs/-/merge_requests/192


Last-update: 2021-10-14
Signed-off-by: Philippe Coval <philippe.coval@huawei.com>