doc: Fix qbs::targetPlatform type
doc: Fix qbs::targetPlatform type

It is string, not stringList

Change-Id: I9844e8e791dfff395dd4af040809c706857aaa28
Reviewed-by: Richard Weickelt <320bca71fc381a4a025636043ca86e734e31cf8b@weickelt.de>
Reviewed-by: Christian Kandeler <3aa99dcdd3f0cac61fb81c2a11771c0cc2497607@qt.io>

qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs