Tweak PR template to indicate placement of issue number

.github/PULL_REQUEST_TEMPLATE.md

@@ -9,8 +9,7 @@
     IF YOUR PULL REQUEST DOES NOT REFERENCE AN ACCEPTED BUG REPORT OR
     FEATURE REQUEST, IT WILL BE MARKED AS INVALID AND CLOSED.
 -->
-### Fixes:
-
+### Fixes: <ISSUE NUMBER GOES HERE>
 <!--
     Please include a summary of the proposed changes below.
 -->