Update elastic-agent-logging.md (#4483)

The calculation of the amount of disk had a small mistake on line 70. Changed:

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

Co-authored-by: Pierre HILBERT <pierre.hilbert@elastic.co>
Co-authored-by: Julien Lind <julien.lind@elastic.co>
(cherry picked from commit 1eb5e12)

docs/elastic-agent-logging.md

@@ -77,5 +77,5 @@ receives.
 
 ## Total footprint
 Given the two log outputs and their default log rotation policies, the
-Elastic-Agent needs about 210Mb (20Mb x 7 + 10Mb + 7 = 210Mb) of disk
+Elastic-Agent needs about 210Mb (20Mb x 7 + 10Mb x 7 = 210Mb) of disk
 for logging.