chore: slim font size for section title in PR template

Keep all section titles in same font size.

.github/PULL_REQUEST_TEMPLATE.md

@@ -30,13 +30,13 @@ Please delete options that are not relevant.
 - [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
 - [ ] This change requires a documentation update
 
-# How Has This Been Tested?
+## How Has This Been Tested?
 
 Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration
 
 - [ ] Test A
 
-# Checklist:
+## Checklist:
 
 - [ ] Followed the style guidelines of this project
 - [ ] Unit tests have been added