Update CHANGELOG.md (influxdata#2519)

* Update CHANGELOG.md

fix link for issue influxdata#2474 -- was pointing to 2491

* chore: Update Changelog

Co-authored-by: J. Emrys Landivar <[email protected]>

CHANGELOG.md

@@ -1,16 +1,16 @@
 # Changelog
 
-## Unreleased
+## v1.5.9 [2021-04-01]
 
 ### Bugfixes
-- [#2479](https://github.com/influxdata/kapacitor/pull/2479): Fix: influx gzip writes for large writes.
+- [#2479](https://github.com/influxdata/kapacitor/pull/2479): Fix influx gzip writes for large writes.
 - [#2488](https://github.com/influxdata/kapacitor/pull/2488): Fix function node name for ServiceNow handler so it is properly camelcased. 
 - [#2489](https://github.com/influxdata/kapacitor/pull/2489): Fix memory leaks in JoinNode and UnionNode.
 - [#2498](https://github.com/influxdata/kapacitor/pull/2498): Avoid infinite hang when closing Kakfa writer, this also prevents the timeout error on an http update to Kafka config.
 
 ### Features
-- [#2472](https://github.com/influxdata/kapacitor/pull2472): Send full event payload on pagerduty resolve, thanks @asvinours!
-- [#2474](https://github.com/influxdata/kapacitor/pull/2491): Add barrier handling to FlattenNode.
+- [#2472](https://github.com/influxdata/kapacitor/pull/2472): Send full event payload on pagerduty resolve, thanks @asvinours!
+- [#2474](https://github.com/influxdata/kapacitor/pull/2474): Add barrier handling to FlattenNode.
 - [#2475](https://github.com/influxdata/kapacitor/pull/2475): Added default color theme to teams alerts, thanks @NoamShaish!
 - [#2491](https://github.com/influxdata/kapacitor/pull/2491): Fix TICKScript AST for bigpanda.