net-mgmt/prometheus2: Fix a path in pkg-plist.

Fix the following installation phase error:

cp: /usr/local/etc/newsyslog.conf.d/prometheus.conf: No such file or directory

It was caused by a relative path being specified in pkg-plist instead of an
absolute one (a leading "/" was missing).

PR: 231548
Reviewed by: dor, krion
Approved by: dor (maintainer), krion (mentor)
Sponsored by: Bally Wulff Games & Entertainment GmbH
Differential Revision: https://reviews.freebsd.org/D17273