docs: dev-tools: checkpatch.rst: Fix grammar

- s/exists/exist/
- s/maybe/may be/

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240208152039.65293-1-thorsten.blum@toblux.com