Fix listener documentation.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.04%. Comparing base (30f9a97) to head (252c25e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #88 +/- ## ======================================= Coverage 89.04% 89.04% ======================================= Files 36 36 Lines 2902 2902 Branches 355 355 ======================================= Hits 2584 2584 Misses 267 267 Partials 51 51 ``` | [Flag](https://app.codecov.io/gh/pymeasure/pyleco/pull/88/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pymeasure) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/pymeasure/pyleco/pull/88/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pymeasure) | `89.04% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pymeasure#carryforward-flags-in-the-pull-request-comment) to find out more.

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