Fix log message

[sed-i]

This PR fixes a log message.
Before, we were using the abstract class directly, Rules.rule_type,

cos-lib/src/cosl/rules.py

Line 238 in d8d7f33

logger.debug("Reading %s rule from %s", Rules.rule_type, file_path)

which resulted in output such as:

Reading <property object at 0x7f1276e21590> rule from /var/lib/juju/agents/unit-prometheus-0/charm/src/prometheus_alert_rules/prometheus_tsdb_wal_truncations_failed.rule

Should use self.