Update ExportProjectDialog.cpp

Fix for MP3 stereo mode doesn't match to the selection in export dialog LMMS#5876

Updated the code to match the LMMS#5876.

src/gui/ExportProjectDialog.cpp

@@ -142,11 +142,11 @@ OutputSettings::StereoMode mapToStereoMode(int index)
 	switch (index)
 	{
 	case 0:
-		return OutputSettings::StereoMode_Stereo;
+		return OutputSettings::StereoMode_Mono;
 	case 1:
-		return OutputSettings::StereoMode_JointStereo;
+ 		return OutputSettings::StereoMode_Stereo;
 	case 2:
-		return OutputSettings::StereoMode_Mono;
+		return OutputSettings::StereoMode_JointStereo;
 	default:
 		return OutputSettings::StereoMode_Stereo;
 	}