contributing: remove reference to DCO

It doesn't exist.

License: MIT
Signed-off-by: Steven Allen <steven@stebalien.com>

contribute.md

@@ -68,9 +68,9 @@ properly written. It checks the following:
   where "User Name" is the author's real (legal) name and
   email@address is one of the author's valid email addresses.
 
-  These trailers mean that the author agrees with the 
-  [developer certificate of origin](docs/developer-certificate-of-origin)
-  and with licensing the work under the [MIT license](docs/LICENSE).
+  These trailers mean that the author agrees to license the work under
+  the [MIT license](docs/LICENSE) and that, to the best of their knowledge, they
+  are able to do so.
 
   To help you automatically add these trailers, you can run the
   [setup_commit_msg_hook.sh](https://github.com/raw/ipfs/community/master/dev/hooks/setup_commit_msg_hook.sh)