[docs] Mistake in urllib.parse docs

[Bo Bayles]

The Python 2.7 docs have this example for urljoin:

>>> urljoin('https://www.cwi.nl/%7Eguido/Python.html',
...         '//www.python.org/%7Eguido')
'http://www.python.org/%7Eguido'


This is incorrect; the result is actually
'https://www.python.org/%7Eguido' - notice https instead of http. This
behavior is what the Note in which the example appears is trying to
explain, so the mistake is worse than it might otherwise be.

The Python 3.6 docs get this right, so they shouldn't need to be
updated in the same way.

Many thanks!
-Bo