Use Heading-4 for PR template for screen-readers. (#1999)

To better integrate with GitHub's PR page, our PR template
should result in h4 elements rather than h1.

See https://marijkeluttekes.dev/blog/articles/2024/08/19/quick-tip-use-h4-in-github-issue-descriptions/

Author: tim-schilling

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,12 +1,12 @@
-# Description
+#### Description
 
 Please include a summary of the change and which issue is fixed. Please also
 include relevant motivation and context. Your commit message should include
 this information as well.
 
 Fixes # (issue)
 
-# Checklist:
+#### Checklist:
 
 - [ ] I have added the relevant tests for this change.
 - [ ] I have added an item to the Pending section of ``docs/changes.rst``.