Update README.MD, replace gitbucket-announce-plugin.jar by gitbucket-h2-backup.jar

Author: garbagetown

README.MD

@@ -58,7 +58,7 @@ sbt clean package
 
 ## Installation
 
-- copy gitbucket-announce-plugin.jar into GITBUCKET_HOME/plugins
+- copy gitbucket-h2-backup.jar into GITBUCKET_HOME/plugins
 - restart gitbucket
 
 ## Release Notes