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: https://github.com/nodejs/node/issues/36338

PR-URL: https://github.com/nodejs/node/pull/36440


Reviewed-By: James M Snell <jasnell@gmail.com>