docs: change PR reference label in template

Author: mtrezza

.github/pull_request_template.md

@@ -11,7 +11,7 @@
 ### Issue Description
 <!-- Add a brief description of the issue this PR solves. -->
 
-Related issue: FILL_THIS_OUT
+Closes: FILL_THIS_OUT
 
 ### Approach
 <!-- Add a description of the approach in this PR. -->