Fixed persistence of user tracking mode in iOS

This typo prevented the tracking mode from being restored when relaunching iosapp.

platform/darwin/settings_nsuserdefaults.mm

@@ -30,7 +30,7 @@ void Settings_NSUserDefaults::load()
     pitch     = [settings[@"pitch"]     doubleValue];
     debug     = [settings[@"debug"]     boolValue];
     
-    unsigned uncheckedTrackingMode = [settings[@"trackingMode"] unsignedIntValue];
+    unsigned uncheckedTrackingMode = [settings[@"userTrackingMode"] unsignedIntValue];
     if (uncheckedTrackingMode > MGLUserTrackingModeNone &&
         uncheckedTrackingMode <= MGLUserTrackingModeFollowWithCourse)
     {
@@ -57,4 +57,4 @@ void Settings_NSUserDefaults::save()
 void Settings_NSUserDefaults::clear()
 {
     [NSUserDefaults resetStandardUserDefaults];
-}
+}