Note new file name in benchmark docs

[ranocha]

@vchuravy noted that the file name changed since writing the docs.

[github-actions]

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[vchuravy]

I was wondering if we should use the new filename in the trixi_include?
But since the rest of the code assumes that the reader checked specific Git SHAs out,
this is more like a blog and not living documentation.

[ranocha]

Yeah, that was my intention - today, we would also get some different results due to later changes.