documentation: s3.md - clarify multipart uploads memory usage

[SublimePeace]

What is the purpose of this change?

Clarify multipart uploads memory usage phrasing. Fix typo.

Was the change discussed in an issue or in the forum before?

#8525

Checklist

• [x] I have read the contribution guidelines.
• [x] I have added tests for all changes in this PR if appropriate.
• [x] I have added documentation for the changes if appropriate.
• [x] All commit messages are in house style.
• [x] I'm done, this Pull Request is ready for review :-)

[SublimePeace]

Oh yea for sure. Fixed.

What do you think about denoting that each flag takes a corresponding number value? Part of me thinks it's redundant, but part of me thinks the equation isn't correct without it.

Maybe something like this:

Multipart uploads will use extra memory equal to: --transfers <number> × --s3-upload-concurrency <number> × --s3-chunk-size <size>. Single part uploads do not use extra memory.

[roucc]

Thanks! I'll merge this now. I think it's fine without the type as it's relatively implicit.