Docs: fix ComboBox snippet
Docs: fix ComboBox snippet

qdoc actually parses the code between \qml and \endqml. there
should be one and only one root element, so move ListModel
definition inline to make qdoc happy.

ComboBox.qml:78: warning:
Unable to parse QML snippet: "Syntax error" at line 7, column 1

Change-Id: I94a4ac8bb91b2fd639c53a1574f436d50eba0096
Reviewed-by: Caroline Chao <cb6a0059f4e516e853a34cb558c25ca5a9bcf4ab@digia.com>

richardmg/qtcomponents,drgvond/qtquickcontrols,kobolabs/qtquickcontrols,kobolabs/qtquickcontrols,mizukusak/qtquickcontrols,drgvond/qtquickcontrols,richardmg/qtcomponents,jfaust/qtquickcontrols,jfaust/qtquickcontrols,mizukusak/qtquickcontrols,mizukusak/qtquickcontrols,drgvond/qtquickcontrols,richardmg/qtcomponents,kobolabs/qtquickcontrols,jfaust/qtquickcontrols