Update docs.asciidoc

[rseldner]

What does this PR do?

Fixed a small typo in the Metricbeat system module documentation where Filebeat is referenced instead of Metricbeat

Why is it important?

The document should reference the correct product

Checklist

• My code follows the style guidelines of this project
  [ ] I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
  [ ] I have made corresponding change to the default configuration files
  [ ] I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @rseldner? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2023-07-12T04:27:36.049+0000

• Duration: 8 min 54 sec

❕ Flaky test report

No test was executed to be analysed.

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