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

tools/tools/git/hooks/prepare-commit-msg

@@ -52,6 +52,7 @@ $(awk '1;/^#$/{exit}' $1)
 # MFC after:	<N [day[s]|week[s]|month[s]].  Request a reminder email>
 # Relnotes:	<Set to 'yes' for mention in release notes.>
 # Security:	<Vulnerability reference (one per line) or description.>
+# Event:	<If this was done at a conference.>
 # Sponsored by:	<If the change was sponsored by an organization.>
 # Pull Request:	<https://github.com/freebsd/<repo>/pull/###>
 # Differential Revision:	<https://reviews.freebsd.org/D###>