.github: clean up PR template (#6050)

I got tired of seeing the literal phrase "Closes #XXX" left in PR bodies. 

Also, this template isn't usually viewed as rendered markdown, so I've removed the markdown formatting and the "Description" heading (which usually gets deleted anyways).

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,7 +1,7 @@
-## Description
+Please add a description of the changes that this PR introduces and the files that
+are the most critical to review.
 
-_Please add a description of the changes that this PR introduces and the files that
-are the most critical to review._ 
+If this PR fixes an open Issue, please include "Closes #XXX" (where "XXX" is the Issue number) 
+so that GitHub will automatically close the Issue when this PR is merged.
 
-Closes: #XXX