prepare-commit-msg: add 'Event'

Add 'Event' to the pre-commit template.

The key word is already documented at
https://docs.freebsd.org/en/articles/committers-guide/#_include_appropriate_metadata_in_a_footer

Suggested by: imp@
Event: BSDCan 2023
Differential Revision: https://reviews.freebsd.org/D40184