[PyQt] Small documentation issue

[David Cortesi]

In the riverbank class reference, the automatically converted QStyle page
(http://riverbankcomputing.co.uk/static/Docs/PyQt4/html/qstyle.html)
refers to several classes that don't exist (QMacStyle, QWindowsStyle, etc)
neither as doc pages (404 error) nor in PyQt4.QtGui.

What does exist is QStyleFactory, which will create some (but not all)
of the QStyle platform versions. E.g. QPlastiqueStyle can be
obtained by QStyleFactory(u'Plastique').