Added etcdv3 maintainer

Author: pkolyvas

CODEOWNERS

@@ -7,7 +7,7 @@
 /backend/remote-state/consul        @hashicorp/consul
 /backend/remote-state/cos           @likexian
 /backend/remote-state/etcdv2        Unmaintained
-/backend/remote-state/etcdv2        Unmaintained
+/backend/remote-state/etcdv2        @bmcstdio
 /backend/remote-state/gcs           @hashicorp/terraform-google
 /backend/remote-state/http          Unmaintained
 /backend/remote-state/manta         Unmaintained