update design doc

Author: polefishu

doc/design/zh/cluster_scaling.md

@@ -15,17 +15,20 @@ numSlots=16384/集群节点。
 待迁移slot数量 * (该源节点负责的slot数量 / slot总数)
 
 当前 Master Slots 分布:
+
 Master[0] -> Slots 0 - 5460      slots=5461
 Master[1] -> Slots 5461 - 10922  slots=5462
 Master[2] -> Slots 10923 - 16383 slots=5461
 
 加入节点 Master[3]，numSlots=16384/4=4096
 那么分配到集群现有每个 Master 节点的待迁移 migratingSlots 数为：
+
 Master[0] migratingSlots = 4096 * (5461 / 16384) = 1365.25=1365
 Master[1] migratingSlots = 4096 * (5462 / 16384) = 1365.5=1366
 Master[2] migratingSlots = 4096 * (5461 / 16384) = 1365.25=1365
 
 最终:
+
 Master[0] -> Slots 1365-5460                      slots=4096
 Master[1] -> Slots 6827-10922                     slots=4096
 Master[2] -> Slots 12288-16383                    slots=4096