Update CHANGELOG.md

This change aligns the indentation as changelog in CLI https://github.com/aws/aws-parallelcluster/blob/release-3.7/CHANGELOG.md

Author: hanwen-pcluste

CHANGELOG.md

@@ -5,11 +5,11 @@ This file is used to list changes made in each version of the AWS ParallelCluste
 
 3.7.1
 ------
+
 **CHANGES**
 - Upgrade Slurm to 23.02.5 (from 23.02.4).
-- Upgrade Pmix to 4.2.6 (from 3.2.3).
-- Upgrade libjwt to 1.15.3 (from 1.12.0).
-
+  - Upgrade Pmix to 4.2.6 (from 3.2.3).
+  - Upgrade libjwt to 1.15.3 (from 1.12.0).
 
 3.7.0
 ------