Refine backend-cluster.md (#7009)
Refine backend-cluster.md (#7009)


OpenSkywalking/skywalking,apache/skywalking,apache/skywalking,ascrutae/sky-walking,ascrutae/sky-walking,OpenSkywalking/skywalking,apache/skywalking,ascrutae/sky-walking,apache/skywalking,ascrutae/sky-walking,apache/skywalking,apache/skywalking,apache/skywalking