Adjust the headings in the PR template

.github/pull_request_template.md

@@ -1,11 +1,11 @@
 <!-- Thank you for your contribution! -->
-### Changes
+## Changes
 
 Fixes #. <!-- Provide issue number if exists -->
 
 <!-- Please give a short brief about these changes. -->
 
-### Checklist
+## Checklist
 
 If this is a user-facing code change, like a bugfix or a new feature, please ensure that
 you've fulfilled the following conditions (where applicable):
@@ -18,7 +18,7 @@ features)
 If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
 these instructions.
 
-## Updating the changelog
+### Updating the changelog
 
 If there are no entries after the last release, use `**UNRELEASED**` as the version.
 If, say, your patch fixes issue #123, the entry should look like this: