fix a typo about watchManagerName in the zookeeperAdmin.md

* *watchManaggerName* :
  (Java system property only: **zookeeper.watchManagerName**)
  **New in 3.6.0:** Added in [ZOOKEEPER-1179](https://issues.apache.org/jira/browse/ZOOKEEPER-1179)
   New watcher manager WatchManagerOptimized is added to optimize the memory overhead in heavy watch use cases. This
   config is used to define which watcher manager to be used. Currently, we only support WatchManager and
   WatchManagerOptimized.