Update notary maintainer (TBD for now)

library/notary

@@ -1,4 +1,4 @@
-Maintainers: Justin Cormack (@justincormack)
+Maintainers: TBD (@docker)
 GitRepo: https://github.com/docker/notary-official-images.git
 GitCommit: 77b9b7833f8dd6be07104b214193788795a320ff
 Architectures: amd64, arm32v6, arm64v8, i386, ppc64le, s390x