message: Update PULL_REQUEST_TEMPLATE.md
  Use `closes` keyword, which will link PR with issue and close it as soon as code will be merged to master branch
diff: <!-- If you are changing just the docs you can create PR directly to master -->
  - [ ] I have changed my target branch to **develop** :facepunch:
  -### Issue # <!-- Please refer issue number here, if exists -->
  +### Closes # <!-- Please refer issue number here, if exists -->
  ## Description
  