[8.9](backport #36478) [docs][ingest-processors] Updated Example in Docs of append processor

[mergify]

This is an automatic backport of pull request #36478 done by Mergify.

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[elasticmachine]

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• Start Time: 2023-09-08T16:47:39.890+0000

• Duration: 8 min 6 sec

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[botelastic]

Hi!
We just realized that we haven't looked into this PR in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
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