[docs] [issue15899] howto/unicode.rst doctest corrections

[Roundup Robot]

Roundup Robot added the comment:

New changeset 9c666683e309 by Ezio Melotti in branch '3.2':
#15949, #15899: use \ufffd instead of ?.
http://hg.python.org/cpython/rev/9c666683e309

New changeset 1aebae2f68da by Ezio Melotti in branch 'default':
#15949, #15899: merge with 3.2.
http://hg.python.org/cpython/rev/1aebae2f68da

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<http://bugs.python.org/issue15899>
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