[issue19273] Update PCbuild/readme.txt

[Roundup Robot]

Roundup Robot added the comment:

New changeset e8307cf23af4 by Larry Hastings in branch 'default':
Issue #19273: The marker comments Argument Clinic uses have been changed
http://hg.python.org/cpython/rev/e8307cf23af4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue19273>
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