fix: improve the pull request template (#439)

Specifically, don't say the description is optional and make sure
there is a handy link to the issue tracker.

PULL_REQUEST_TEMPLATE.md

@@ -1,9 +1,11 @@
 ## Checklist
 
 - [ ] I have read the [contribution guidelines](https://github.com/ooni/probe-cli/blob/master/CONTRIBUTING.md)
-- [ ] reference issue for this pull request: REFERENCE_ISSUE_URL
-- [ ] related ooni/spec pull request: RELATED_OONI_SPEC_PULL_REQUEST_URL
+- [ ] reference issue for this pull request: `REFERENCE_ISSUE_URL`
+- [ ] related ooni/spec pull request: `RELATED_OONI_SPEC_PULL_REQUEST_URL`
+
+Location of the issue tracker: https://github.com/ooni/probe
 
 ## Description
 
-Insert here a more detailed description (if applicable).
+Please, insert here a more detailed description.