ZOOKEEPER-3745: Update copyright notices from 2019 to 2020

Author: tison <wander4096@gmail.com>

Reviewers: Enrico Olivelli <eolivelli@apache.org>

Closes #1278 from TisonKun/ZOOKEEPER-3745

NOTICE.txt

@@ -1,5 +1,5 @@
 Apache ZooKeeper
-Copyright 2009-2019 The Apache Software Foundation
+Copyright 2009-2020 The Apache Software Foundation
 
 This product includes software developed at
 The Apache Software Foundation (http://www.apache.org/).

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

@@ -1,5 +1,5 @@
 <!--
-Copyright 2002-2019 The Apache Software Foundation
+Copyright 2002-2020 The Apache Software Foundation
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

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

@@ -1,5 +1,5 @@
 <!--
-Copyright 2002-2019 The Apache Software Foundation
+Copyright 2002-2020 The Apache Software Foundation
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -92,4 +92,4 @@ More details can be found in [here](http://zookeeper.apache.org/doc/current/zook
 
 <a name="four-letter-words"></a>
 ## Four letter words
-More details can be found in [here](http://zookeeper.apache.org/doc/current/zookeeperAdmin.html#sc_zkCommands)
+More details can be found in [here](http://zookeeper.apache.org/doc/current/zookeeperAdmin.html#sc_zkCommands)

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

@@ -1,5 +1,5 @@
 <!--
-Copyright 2002-2019 The Apache Software Foundation
+Copyright 2002-2020 The Apache Software Foundation
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

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

@@ -1,5 +1,5 @@
 <!--
-Copyright 2002-2019 The Apache Software Foundation
+Copyright 2002-2020 The Apache Software Foundation
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -382,4 +382,4 @@ limitations under the License.
 - [35] http://atlas.apache.org/HighAvailability.html
 - [36] http://griffin.apache.org/docs/usecases.html
 - [37] https://fluo.apache.org/
-- [38] https://spring.io/projects/spring-cloud-zookeeper
+- [38] https://spring.io/projects/spring-cloud-zookeeper

zookeeper-server/src/main/resources/NOTICE.txt

@@ -1,5 +1,5 @@
 Apache ZooKeeper
-Copyright 2009-2019 The Apache Software Foundation
+Copyright 2009-2020 The Apache Software Foundation
 
 This product includes software developed at
 The Apache Software Foundation (http://www.apache.org/).