Fix broken OWNER links in sig-etcd README

Fixed active projects' OWNER link only, mainly by changing file name 
from MAINTAINERS to OWNERS and updating branch name from master to main.

Notice that for a repo that doesn't have a MAINTAINER or OWNER file, 
the link is left as-is, the `dbtester` repo.

References:
- etcd-io/bbolt#578

sig-etcd/README.md

@@ -49,11 +49,11 @@ The following [subprojects][subproject-definition] are owned by sig-etcd:
 ### auger
 Directly access data objects stored in etcd by kubernetes.
 - **Owners:**
-  - [etcd-io/auger](https://github.com/etcd-io/auger/blob/master/OWNERS)
+  - [etcd-io/auger/OWNERS](https://github.com/etcd-io/auger/blob/master/OWNERS)
 ### bbolt
 An embedded key/value database for Go.
 - **Owners:**
-  - [etcd-io/bbolt/MAINTAINERS](https://github.com/etcd-io/bbolt/blob/master/MAINTAINERS)
+  - [etcd-io/bbolt/OWNERS](https://github.com/etcd-io/bbolt/blob/main/OWNERS)
 ### cetcd
 Serve Consul with etcd
 - **Owners:**
@@ -65,15 +65,15 @@ Distributed database benchmark tester
 ### discovery.etcd.io
 Kubernetes manifests powering discovery.etcd.io
 - **Owners:**
-  - [etcd-io/discovery.etcd.io/MAINTAINERS](https://github.com/etcd-io/discovery.etcd.io/blob/master/MAINTAINERS)
+  - [etcd-io/discovery.etcd.io/OWNERS](https://github.com/etcd-io/discovery.etcd.io/blob/master/OWNERS)
 ### discoveryserver
 Public etcd Discovery Service
 - **Owners:**
-  - [etcd-io/discoveryserver/MAINTAINERS](https://github.com/etcd-io/discoveryserver/blob/master/MAINTAINERS)
+  - [etcd-io/discoveryserver/OWNERS](https://github.com/etcd-io/discoveryserver/blob/master/OWNERS)
 ### etcd
 Distributed reliable key-value store for the most critical data of a distributed system
 - **Owners:**
-  - [etcd-io/etcd/MAINTAINERS](https://github.com/etcd-io/etcd/blob/master/MAINTAINERS)
+  - [etcd-io/etcd/OWNERS](https://github.com/etcd-io/etcd/blob/main/OWNERS)
 ### etcd-play
 etcd playground
 - **Owners:**
@@ -85,31 +85,31 @@ etcd playground
 ### gofail
 failpoints for go
 - **Owners:**
-  - [etcd-io/gofail/MAINTAINERS](https://github.com/etcd-io/gofail/blob/master/MAINTAINERS)
+  - [etcd-io/gofail/OWNERS](https://github.com/etcd-io/gofail/blob/master/OWNERS)
 ### govanityurls
 Use a custom domain in your Go import path
 - **Owners:**
-  - [etcd-io/govanityurls/MAINTAINERS](https://github.com/etcd-io/govanityurls/blob/master/MAINTAINERS)
+  - [etcd-io/govanityurls/OWNERS](https://github.com/etcd-io/govanityurls/blob/master/OWNERS)
 ### jetcd
 etcd java client
 - **Owners:**
-  - [etcd-io/jetcd/MAINTAINERS](https://github.com/etcd-io/jetcd/blob/master/MAINTAINERS)
+  - [etcd-io/jetcd/OWNERS](https://github.com/etcd-io/jetcd/blob/main/OWNERS)
 ### maintainers
 issue tracking for project wide non-code concerns
 - **Owners:**
   - [etcd-io/maintainers/MAINTAINERS](https://github.com/etcd-io/maintainers/blob/master/MAINTAINERS)
 ### protodoc
 protodoc generates Protocol Buffer documentation.
 - **Owners:**
-  - [etcd-io/protodoc/MAINTAINERS](https://github.com/etcd-io/protodoc/blob/master/MAINTAINERS)
+  - [etcd-io/protodoc/OWNERS](https://github.com/etcd-io/protodoc/blob/master/OWNERS)
 ### raft
 Raft library for maintaining a replicated state machine
 - **Owners:**
-  - [etcd-io/raft/MAINTAINERS](https://github.com/etcd-io/raft/blob/master/MAINTAINERS)
+  - [etcd-io/raft/OWNERS](https://github.com/etcd-io/raft/blob/main/OWNERS)
 ### website
 etcd-io
 - **Owners:**
-  - [etcd-io/website/MAINTAINERS](https://github.com/etcd-io/website/blob/master/MAINTAINERS)
+  - [etcd-io/website/OWNERS](https://github.com/etcd-io/website/blob/main/OWNERS)
 ### zetcd
 Serve the Apache Zookeeper API but back it with an etcd cluster
 - **Owners:**