changelog: set date for 5.3.0 release

CHANGELOG.md

@@ -4,7 +4,7 @@
 
 * **Datasource Proxy**: Keep trailing slash for datasource proxy requests [#13326](https://github.com/grafana/grafana/pull/13326), thx [@ryantxu](https://github.com/ryantxu)
 
-# 5.3.0 (unreleased)
+# 5.3.0 (2018-10-10)
 
 * **Stackdriver**: Filter wildcards and regex matching are not yet supported [#13495](https://github.com/grafana/grafana/issues/13495)
 * **Stackdriver**: Support the distribution metric type for heatmaps [#13559](https://github.com/grafana/grafana/issues/13559)