Fix code style of example in Wagtail API docs

[Stormheg]

Example code should be compliant with PEP-8.

Noticed this when reading this section of the docs and decided to volunteer a few minutes to fix it 😁

Looks like the class names were accidentally changed to lowercase when this section was converted to markdown in 7e73349.

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Should we perhaps consider adding some sort of formatter / linter for the documentation examples to ensure our examples follow the same code style?

[squash-labs]

Manage this branch in Squash

Test this branch here: https://stormheg-patch-1-u8g9z.squash.io

[lb-]

Thanks @Stormheg - will pull this into 4.0.2 as it was a 4.0 docs regression.

[lb-]

Should we perhaps consider adding some sort of formatter / linter for the documentation examples to ensure our examples follow the same code style?

@Stormheg - could you add this scenario and thoughts to the existing similar discussion about this #8753 (it is about prettier adoption in Markdown, but this is a similar underlying problem).

[lb-]

Cherry picked to 4.0 stable 4b0ac7e