message: docs: fix link in pull request template
diff: For an explanation of how to fill out the fields, please see the relevant section
  -in [PULL_REQUESTS.md](./PULL_REQUESTS.md)
  +in [PULL_REQUESTS.md](https://github.com/envoyproxy/envoy/blob/master/PULL_REQUESTS.md)
  Description:
  Risk Level:
  