CHANGELOG: rename to "CHANGELOG-2.3.md"

CHANGELOG-2.7.md → CHANGELOG-2.3.md

@@ -6,4 +6,4 @@ See [code changes](https://github.com/coreos/etcd/compare/v2.3.7...v2.3.8).
 
 ### Changed
 
-- Compile with `Go 1.7.5`.
+- Compile with `Go 1.7.5`.