doc: update PR checklist

`signs_ui` now incorporates this repo's `signs.json` via a dependency
instead of a copy of the file.

.github/PULL_REQUEST_TEMPLATE.md

@@ -9,5 +9,5 @@
 - [ ] Meets ticket's acceptance criteria
 - [ ] Any new or changed functions have typespecs
 - [ ] Tests were added for any new functionality (don't just rely on Codecov)
-- [ ] `signs.json` changes were also made in [signs_ui](https://github.com/mbta/signs_ui/blob/master/priv/signs.json)
+- [ ] If `signs.json` was changed, there is a follow-up PR to `signs_ui` to update its dependency on this project
 - [ ] This branch was deployed to the staging environment and is currently running with no unexpected increase in warnings, and no errors or crashes (compare on Splunk: [staging](https://mbta.splunkcloud.com/en-US/app/search/search?q=search%20index%3Drealtime-signs-dev%20%22%5Berror%5D%22%20OR%20%22%5Bwarn%5D%22%20OR%20%22CRASH%22&display.page.search.mode=verbose&dispatch.sample_ratio=1&earliest=-4h%40m&latest=now&sid=1545840107.3874236) vs. [prod](https://mbta.splunkcloud.com/en-US/app/search/search?q=search%20index%3Drealtime-signs-prod%20%22%5Berror%5D%22%20OR%20%22%5Bwarn%5D%22%20OR%20%22CRASH%22&display.page.search.mode=verbose&dispatch.sample_ratio=1&earliest=-4h%40m&latest=now&sid=1545840137.3874305))