Fix `alert_rules` description

[ioanalazea]

Description

Not a major problem but the description look a bit off (suspecting because of whitespaces):
Thanks!

Charm revision

latest/stable

Juju version

3.6.28

Cloud

No response

Expected behaviour

To look like the other options

Reproduce / Test

Run juju config openstack-exporter

Notes & References

No response

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The internal ticket has been created: https://warthogs.atlassian.net/browse/SOLENG-2317.

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