Update knn-search.asciidoc

[JohannesMahne]

Better formatting for the calculation of off heap memory required.

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[github-actions]

Documentation preview:

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[elasticsearchmachine]

@JohannesMahne please enable the option "Allow edits and access to secrets by maintainers" on your PR. For more information, see the documentation.

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[leemthompo]

This was fixed in another PR. Thanks for opening the PR!