make PR template headings actual markdown headings (#14240)

.github/PULL_REQUEST_TEMPLATE.md

@@ -3,16 +3,22 @@
 - In order to be merged, you must add the most appropriate category Label (Added, Changed, Fixed) to your PR
 -->
 <!-- Explain why this is an improvement (Does this add missing functionality, improve performance, or reduce complexity?) -->
-Purpose:
+### Purpose:
+
 
 
 <!-- Does this PR introduce a breaking change? -->
-Current Behavior:
-New Behavior:
+### Current Behavior:
+
+
+
+### New Behavior:
+
 
 
 <!-- As we aim for complete code coverage, please include details regarding unit, and regression tests -->
-Testing Notes:
+### Testing Notes:
+
 
 
 <!-- Attach any visual examples, or supporting evidence (attach any .gif/video/console output below) -->