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>

contributing/dco.rst

@@ -12,7 +12,7 @@ Overview
 ********
 
 Commits **MUST** be submitted only with a sign-off by the submitter. A commit
-without a sign-off will be automatically rejected. You need not be the author
+without a sign-off will be automatically rejected. You don't need be the author
 or the copyright holder of the contribution, but you must make sure that you
 have the power to submit on behalf of those who are.