docs: minimize PULL_REQUEST_TEMPLATE.md

Problem:
The `-----` h2 markup doesn't look great in the actual git logs. And it
takes up an extra 2 lines.

Solution:
Revert to `##` for the h2 headings.

Author: justinmk3

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,9 +1,7 @@
-Problem
--------
+## Problem
 
 
-Solution
---------
+## Solution
 
 
 ---