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

[Serhiy Storchaka]

Change by Serhiy Storchaka <storchaka+cpython at gmail.com>:


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components: +Tkinter

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue33280>
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