Documentation/op-guide: fix markdown highlight syntax

Signed-off-by: Gyu-Ho Lee <gyuhox@gmail.com>

Documentation/op-guide/clustering.md

@@ -281,7 +281,7 @@ ETCD_DISCOVERY=https://discovery.etcd.io/3e86b59982e49066c5d813af1c2e2579cbf573d
 --discovery https://discovery.etcd.io/3e86b59982e49066c5d813af1c2e2579cbf573de
 ```
 
-**Each member must have a different name flag specified or else discovery will fail due to duplicated names. `Hostname` or `machine-id` can be a good choice. **
+**Each member must have a different name flag specified or else discovery will fail due to duplicated names. `Hostname` or `machine-id` can be a good choice.**
 
 Now we start etcd with those relevant flags for each member: