client: rename "master" branch references to "main"

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

client/v3/doc.go

@@ -61,7 +61,7 @@
 //
 //  1. context error: canceled or deadline exceeded.
 //  2. gRPC error: e.g. when clock drifts in server-side before client's context deadline exceeded.
-//  See https://github.com/etcd-io/etcd/blob/master/api/v3rpc/rpctypes/error.go
+//  See https://github.com/etcd-io/etcd/blob/main/api/v3rpc/rpctypes/error.go
 //
 // Here is the example code to handle client errors:
 //