ZOOKEEPER-4046: Fix typo

Fix typo

Author: KangZhiDong <worldkzd@gmail.com>

Reviewers: maoling maoling1992191@sina.com

Closes #1570 from KangZhiDong/ZOOKEEPER-4046

zookeeper-docs/src/main/resources/markdown/zookeeperUseCases.md

@@ -121,9 +121,9 @@ limitations under the License.
   - ZooKeeper maintains ephemeral cluster membership information. The Drillbits use ZooKeeper to find other Drillbits in the cluster,
     and the client uses ZooKeeper to find Drillbits to submit a query [28].
 
-### [Apache Druid(Incubating)](https://druid.apache.org/)
-  - Apache Druid (incubating) is a high performance real-time analytics database.
-  - Apache Druid (incubating) uses Apache ZooKeeper (ZK) for management of current cluster state. The operations that happen over ZK are [27]:
+### [Apache Druid](https://druid.apache.org/)
+  - Apache Druid is a high performance real-time analytics database.
+  - Apache Druid uses Apache ZooKeeper (ZK) for management of current cluster state. The operations that happen over ZK are [27]:
     - Coordinator leader election
     - Segment "publishing" protocol from Historical and Realtime
     - Segment load/drop protocol between Coordinator and Historical

zookeeper-server/src/main/java/org/apache/zookeeper/server/quorum/LearnerHandler.java

@@ -551,7 +551,7 @@ public class LearnerHandler extends ZooKeeperThread {
             boolean needSnap = syncFollower(peerLastZxid, learnerMaster);
 
             // syncs between followers and the leader are exempt from throttling because it
-            // is importatnt to keep the state of quorum servers up-to-date. The exempted syncs
+            // is important to keep the state of quorum servers up-to-date. The exempted syncs
             // are counted as concurrent syncs though
             boolean exemptFromThrottle = getLearnerType() != LearnerType.OBSERVER;
             /* if we are not truncating or sending a diff just send a snapshot */