fix angle brackets pr template (AMReX-Codes#1215)

## Summary

The `-->` in the html comment part of the PR template closes the comment and leaves the words after that and before the actual closing `-->` visible.  This is fixed.

The proposed changes:
- [ ] fix a bug or incorrect behavior in AMReX
- [ ] add new capabilities to AMReX
- [ ] changes answers in the test suite to more than roundoff level
- [ ] are likely to significantly affect the results of downstream AMReX users
- [ ] are described in the proposed changes to the AMReX documentation, if appropriate

.github/pull_request_template.md

@@ -5,7 +5,7 @@
   3. Provide any background to help reviewers understand the proposed changes
   4. Fill out the checklist (using [x] to check the box)
 
-NOTE: All text between <-- and --> will not be formatted in md previewers
+NOTE: All text between < and > will not be formatted in md previewers
 -->
 
 ## Summary