Fix spelling mistakes in tkinter.py

[bravegnu]

Ran the docstrings through spell checker, and fixed spelling issues.

[the-knights-who-say-ni]

Hello, and thanks for your contribution!

I'm a bot set up to make sure that the project can legally accept your contribution by verifying you have signed the PSF contributor agreement (CLA).

Unfortunately we couldn't find an account corresponding to your GitHub username on bugs.python.org (b.p.o) to verify you have signed the CLA. This is necessary for legal reasons before we can look at your contribution. Please follow the steps outlined in the CPython devguide to rectify this issue.

Thanks again to your contribution and we look forward to looking at it!

[serhiy-storchaka]

Thank you for your patch @bravegnu. Do you mind to backport your change to other active branches (3.6, 3.5 and 2.7)?