doc: Add issue reference to the PR template #36338

Affected url(/sitelet?url=https%3A%2F%2Fgithub.com%2Fnodejs%2Fnode%2Fissues%2Fs):

• https://github.com/nodejs/node/blob/master/doc/guides/contributing/pull-requests.md#step-8-opening-the-pull-request

Description

✍️

The PR template should include a Reference isssue (Fixes #issue_number) so the 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.

• I would like to work on this issue and
  submit a pull request.