[8.7](backport #4483) Update elastic-agent-logging.md with right calculation details for disk sizing

[mergify]

While researching for a case I noticed that there might have been a typo mistake here where the calculation of the default disk size required for the Elastic Agent's log files was explained.

What does this PR do?

• The calculation of the amount of disk had a small mistake on line 70.
• It should be:

(20Mb x 7 + 10Mb x 7 = 210Mb)

Why is it important?

Right calculation of the defaults would be necessary to set the right expectations.

---

This is an automatic backport of pull request #4483 done by Mergify.

[mergify]

Cherry-pick of 1eb5e12 has failed:

On branch mergify/bp/8.7/pr-4483
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	deleted by us:   docs/elastic-agent-logging.md

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