Doc: Fix apidoc for QQuickLoader::sourceComponent

Setting this to an empty string doesn't make sense nor works

Change-Id: I4dd89d0356da30bc46e5fdac827e30b6f9a6a7a4
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>

src/quick/items/qquickloader.cpp

@@ -449,8 +449,7 @@ void QQuickLoader::loadFromSource()
     }
     \endqml
 
-    To unload the currently loaded object, set this property to an empty string
-    or \c undefined.
+    To unload the currently loaded object, set this property to \c undefined.
 
     Since \c {QtQuick 2.0}, Loader is able to load any type of object; it
     is not restricted to Item types.