Add reminder about format-json to PR Checklist (#22055)

docs/pr_checklist.md

@@ -80,6 +80,7 @@ https://github.com/qmk/qmk_firmware/pulls?q=is%3Apr+is%3Aclosed+label%3Akeyboard
  - Encoder Configuration
  - Bootmagic Configuration
  - LED Indicator Configuration
+ - Run `qmk format-json` on this file before submitting your PR. Be sure to append the `-i` flag to directly modify the file, or paste the outputted code into the file. 
 - `readme.md`
  - must follow the [template](https://github.com/qmk/qmk_firmware/blob/master/data/templates/keyboard/readme.md)
  - flash command is present, and has `:flash` at end