Fix FileDialog when saving world (#3110)

Signed-off-by: Ian Chen <ichen@openrobotics.org>

Author: iche033

src/gui/resources/GazeboDrawer.qml

@@ -134,7 +134,7 @@ Rectangle {
     id: saveWorldDialog
     title: "Save world"
     currentFolder: StandardPaths.writableLocation(StandardPaths.HomeLocation)
-    fileMode: FileDialog.OpenFile
+    fileMode: FileDialog.SaveFile
     nameFilters: [ "SDF files (*.sdf)" ]
     onAccepted: {
       saveWorldFileText.text = fileUrl;