Fix typo in Appextras documentation

[fredvd]

The ignore rule filters results from the match pattern, not from itself.

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📚 Documentation preview 📚: https://volto--6083.org.readthedocs.build/

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[sneridagh]

@fredvd could you take a look and confirm that all is good?

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[sneridagh]

@stevepiercy ready when you are.