[docs] [issue33280] Update link to Tcl/Tk 8.6 man pages in tkinter.rst

[Serhiy Storchaka]

Serhiy Storchaka <storchaka+cpython at gmail.com> added the comment:

I suggest to just remove the version suffix "8.6".

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nosy: +serhiy.storchaka, terry.reedy
versions: +Python 2.7, Python 3.6, Python 3.7

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