Change PR template from 1234 to XXX

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,7 +2,7 @@ Follow this checklist to help us incorporate your contribution quickly and easil
 
  - [ ] Make sure there is a [JIRA issue](https://issues.apache.org/jira/projects/BEAM/issues/) filed for the change (usually before you start working on it).  Trivial changes like typos do not require a JIRA issue.  Your pull request should address just this issue, without pulling in other changes.
  - [ ] Each commit in the pull request should have a meaningful subject line and body.
- - [ ] Format the pull request title like `[BEAM-1234] Fixes bug in ApproximateQuantiles`, where you replace `BEAM-1234` with the appropriate JIRA issue.
+ - [ ] Format the pull request title like `[BEAM-XXX] Fixes bug in ApproximateQuantiles`, where you replace `BEAM-XXX` with the appropriate JIRA issue.
  - [ ] Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
  - [ ] Run `mvn clean verify` to make sure basic checks pass. A more thorough check will be performed on your pull request automatically.
  - [ ] If this contribution is large, please file an Apache [Individual Contributor License Agreement](https://www.apache.org/licenses/icla.pdf).