fix(mkdir): Use lower case for arg names

[Tyriar]

Fixes #2589

The originals were from the man page, so it's somewhat ambiguous what the right thing to do here is.

[withfig-bot]

Overview

src/mkdir.ts:

Info:

[withfig-bot]

Hello @Tyriar,
thank you very much for creating a Pull Request!
Here is a small checklist to get this PR merged as quickly as possible:

• Do all subcommands / options which take arguments include the args property (args: {})?
• Are all options modular? E.g. -a -u -x instead of -aux
• Have all other checks passed?

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