CONTRIBUTING: require two people to review on etcd

CONTRIBUTING.md

@@ -21,7 +21,7 @@ This is a rough outline of what a contributor's workflow looks like:
 - Make sure your commit messages are in the proper format (see below).
 - Push your changes to a topic branch in your fork of the repository.
 - Submit a pull request to coreos/etcd.
-- Your PR must receive a LGTM from each maintainer found in the MAINTAINERS file.
+- Your PR must receive a LGTM from two maintainers found in the MAINTAINERS file.
 
 Thanks for your contributions!