Update PR template (vesoft-inc#3809)

Co-authored-by: Sophie <[email protected]>

.github/pull_request_template.md

@@ -3,26 +3,26 @@ Thanks for your contribution!
 In order to review PR more efficiently, please add information according to the template.
 -->
 
-#### What type of PR is this?
+## What type of PR is this?
 - [ ] bug
 - [ ] feature
 - [ ] enhancement
 
-#### What problem(s) does this PR solve?
-Issue(s) number: 
+## What problem(s) does this PR solve?
+#### Issue(s) number: 
 
-Description:
+#### Description:
 
 
-#### How do you solve it?
+## How do you solve it?
 
 
-
-#### Special notes for your reviewer, ex. impact of this fix, design document, etc:
 
+## Special notes for your reviewer, ex. impact of this fix, design document, etc:
 
 
-#### Checklist:
+
+## Checklist:
 Tests:
 - [ ] Unit test(positive and negative cases)
 - [ ] Function test
@@ -36,7 +36,7 @@ Affects:
 - [ ] Performance impacted: Consumes more CPU/Memory
 
 
-#### Release notes:
+## Release notes:
 
 Please confirm whether to be reflected in release notes and how to describe:
 > ex. Fixed the bug .....