Typo in create_dir doc comment

[Dorumin]

changelog: none

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Manishearth (or someone else) soon.

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[Manishearth]

@bors r+ rollup

thanks!

[bors]

📌 Commit 1d6fac6 has been approved by Manishearth

[bors]

⌛ Testing commit 1d6fac6 with merge 5c00931...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Manishearth
Pushing 5c00931 to master...