Fixed the 'os' modules tmpdir function declaration.

Fixed an faulty declaration for the function name. It was called tmpDir() when the function actually is tmpdir():
- http://nodejs.org/api/os.html#os_os_tmpdir

Even took a quick peek into random 0.11.x API documents:
- http://nodejs.org/docs/v0.11.13/api/os.html#os_os_tmpdir