Set the merge date on RFC 0018 stage 2 (#1429)

* set correct advancement date for stage 2

* add pr number

rfcs/text/0018-extend-threat-group-software.md

@@ -2,7 +2,7 @@
 <!-- Leave this ID at 0000. The ECS team will assign a unique, contiguous RFC number upon merging the initial stage of this RFC. -->
 
 - Stage: **2 (candidate)** <!-- Update to reflect target stage. See https://elastic.github.io/ecs/stages.html -->
-- Date: **2021-00-00** <!-- The ECS team sets this date at merge time. This is the date of the latest stage advancement. -->
+- Date: **2021-05-25** <!-- The ECS team sets this date at merge time. This is the date of the latest stage advancement. -->
 
 <!--
 As you work on your RFC, use the "Stage N" comments to guide you in what you should focus on, for the stage you're targeting.
@@ -206,6 +206,7 @@ e.g.:
 * Stage 0: https://github.com/elastic/ecs/pull/1300
 * Stage 1: https://github.com/elastic/ecs/pull/1335
 * Stage 2: https://github.com/elastic/ecs/pull/1395
+    * Stage 2 advancement date correction: https://github.com/elastic/ecs/pull/1429
 
 <!--
 * Stage 1: https://github.com/elastic/ecs/pull/NNN