Fix some rst warnings

[dforsi]

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What does this PR do and why is it necessary?

This fixes two warnings, one that results in an user-visible change, the source of Listing 15 isn't visible in the locally built docs and it's caption shows the warning in the online docs at http://docs.octoprint.org/en/master/plugins/hooks.html#octoprint-comm-protocol-action

How was it tested? How can it be tested by the reviewer?

Comparing the HTML output.

Any background context you want to provide?

What are the relevant tickets if any?

Screenshots (if appropriate)

Before this fix

After this fix

Further notes

[Kunsi]

First of all, you should not create pull requests against master branch here, always create PRs against devel. Then, you should fix your PR overall, since it contains a lot of commits which are not related to your correction.

[dforsi]

Thank you for your review.
I selected the wrong branch when creating this pull request, but in fact I branched off maintenance.

I think this is suitable for merging into maintenance because this is an error in the published developer's documentation, but I will rebase to devel if you think that this is error is too small for maintenance.

[Kunsi]

Looks good from my side. However i'm not one of the maintainers of the project, so i'm not able to merge your changes. I'm just some user trying to take some load off the maintainers.

[foosel]

I'm on the other hand am one of the maintainers, just came back from my vacation and am very thankful for this PR :)