[PR] Improve type of 'mkdir' and 'mkdirSync'
[PR] Improve type of 'mkdir' and 'mkdirSync'

Summary:
See: https://nodejs.org/api/fs.html#fs_fs_mkdir_path_options_callback
Pull Request resolved: https://github.com/facebook/flow/pull/7231

Differential Revision: D13505064

Pulled By: mroch

fbshipit-source-id: 06226d401714859f75d66218ed608700eebed984

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow