CHANGELOG.md: fix changelog to incorporate new fix (#3737)

This commit fixes the order of the changelog to properly reflect a
recent change.

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>

CHANGELOG.md

@@ -12,10 +12,6 @@ We use _breaking :warning:_ to mark changes that are not backward compatible (re
 
 ## Unreleased
 
-### Fixed
-
-- [#3734](https://github.com/thanos-io/thanos/pull/3734) pkg/rules/proxy: fix hotlooping when receiving client errors
-
 ## [v0.18.0](https://github.com/thanos-io/thanos/releases/tag/v0.18.0) - 2021.01.19
 
 ### Added
@@ -60,6 +56,7 @@ We use _breaking :warning:_ to mark changes that are not backward compatible (re
 ### Fixed
 
 - [#3480](https://github.com/thanos-io/thanos/pull/3480) Query Frontend: Fixed regression.
+- [#3734](https://github.com/thanos-io/thanos/pull/3734) pkg/rules/proxy: fix hotlooping when receiving client errors
 
 ### Changed