update example PR in adding_ping_data.md
Created by: arussellsaw
@unknwon mentioned that the example PR didn't include a CHANGELOG.md entry
https://github.com/sourcegraph/sourcegraph/pull/15389#pullrequestreview-523931513
Created by: arussellsaw
@unknwon mentioned that the example PR didn't include a CHANGELOG.md entry
https://github.com/sourcegraph/sourcegraph/pull/15389#pullrequestreview-523931513