[pydotorg-www] Minor typo fix in https://wiki.python.org/moin/UsingSlots (WillHawkins)

[Will Hawkins]

Thank you for the great, comprehensive page about using slots!! I
noticed a small typo near the bottom of the page that I was going to
try to correct:

... or __weak_ref__to be present. According to the ...

I believe that should simply say __weakref__. I'd love to submit a fix
but don't have permission!

Thanks again for the great resource!
Will