MAINTAINERS: update import path to "go.etcd.io"

Signed-off-by: Gyuho Lee <leegyuho@amazon.com>

MAINTAINERS

@@ -14,4 +14,4 @@ Hitoshi Mitake <h.mitake@gmail.com> (@mitake) pkg:*
 Joe Betz <jpbetz@google.com> (@jpbetz) pkg:*
 Xiang Li <xiangli.cs@gmail.com> (@xiang90) pkg:*
 
-Ben Darnell <ben@cockroachlabs.com> (@bdarnell) pkg:github.com/coreos/etcd/raft
+Ben Darnell <ben@cockroachlabs.com> (@bdarnell) pkg:go.etcd.io/etcd/raft