doc: add issue reference to github pr template

The PR template should include a Reference isssue (Fixes #issue_number)
so the PR can be tracked back to the issue easily.For a beginner trying
to read PRs to become comfortable with the codebase,it is really helpful
if one can directly reach the issue the PR fixes.

Fixes: nodejs#36338

.github/PULL_REQUEST_TEMPLATE.md

@@ -7,6 +7,10 @@ Bug fixes and new features should include tests and possibly benchmarks.
 Contributors guide: https://github.com/nodejs/node/blob/master/CONTRIBUTING.md
 -->
 
+#### Related Issues
+
+Fixes: https://github.com/nodejs/node/issues/<issue_number>
+
 ##### Checklist
 <!-- Remove items that do not apply. For completed items, change [ ] to [x]. -->
 

doc/guides/contributing/pull-requests.md

@@ -281,6 +281,10 @@ Bug fixes and new features should include tests and possibly benchmarks.
 Contributors guide: https://github.com/nodejs/node/blob/master/CONTRIBUTING.md
 -->
 
+#### Related Issues
+
+Fixes: https://github.com/nodejs/node/issues/<issue_number>
+
 #### Checklist
 <!-- Remove items that do not apply. For completed items, change [ ] to [x]. -->