fix: don't use private Harness names (trivial)

[tonyandrewmeyer]

Overview

Swaps use of harness._charm for harness.charm.

Rationale

Private names (ones with leading underscores) of dependencies, including ops, should not be used. These may change at any time, and are not intended for public consumption. In this case, it's of no value, as the charm is publicly exposed via a method on the harness.

Juju Events Changes

N/A

Module Changes

N/A

Library Changes

N/A

Checklist

• [x] The charm style guide was applied
• [x] The contributing guide was applied
• [x] The changes are compliant with ISD054 - Manging Charm Complexity
• [x] The documentation is generated using src-docs
• [x] The documentation for charmhub is updated.
• [x] The PR is tagged with appropriate label (urgent, trivial, complex)