clientv3/concurrency: remove the unneeded slash
clientv3/concurrency: remove the unneeded slash

Since NewMutex will append a slash to pfx, there is no need to append a
slash beforehand.

mkumatag/etcd,ajityagaty/etcd,cockroachdb/etcd,rnaveiras/etcd,cockroachdb/etcd,mqliang/etcd,mkumatag/etcd,mqliang/etcd,li-ang/etcd,gyuho/etcd,mkumatag/etcd,li-ang/etcd,ajityagaty/etcd,mqliang/etcd,ajityagaty/etcd,etcd-io/etcd,dannysauer/etcd,etcd-io/etcd,coreos/etcd,etcd-io/etcd,coreos/etcd,rnaveiras/etcd,dannysauer/etcd,rnaveiras/etcd,gyuho/etcd