Set the merge date on RFC 0015 stage 2 (#1409)

* set correct advancement date for stage 2

* add PR number

rfcs/text/0015-create-file-elf.md

@@ -1,7 +1,7 @@
 # 0015: Create the ELF sub-field of the File fieldset
 
 - Stage: **2 (candidate)**
-- Date: **2021-02-10**
+- Date: **2021-05-04**
 
 Create the Executable Linkable Format (ELF) sub-field, of the `file` top-level fieldset. This document metadata can be used for malware research, as well as coding and other application development efforts.
 
@@ -235,6 +235,7 @@ The following are the people that consulted on the contents of this RFC.
 
 * Stage 1: https://github.com/elastic/ecs/pull/1077
 * Stage 2: https://github.com/elastic/ecs/pull/1294
+  * Stage 2 advancement date correction: https://github.com/elastic/ecs/pull/1409
 
 <!--
 * Stage 1: https://github.com/elastic/ecs/pull/NNN