chore: Fix the changelog format

Author: jdneo

CHANGELOG.md

@@ -3,7 +3,7 @@ All notable changes to the "leetcode" extension will be documented in this file.
 
 Check [Keep a Changelog](http://keepachangelog.com/) for recommendations on how to structure this file.
 
-##[0.15.2]
+## [0.15.2]
 ### Added
 - Prompt to open the workspace for LeetCode [#130](https://github.com/jdneo/vscode-leetcode/issues/130)
 - Support deleting sessions [#198](https://github.com/jdneo/vscode-leetcode/issues/130)