[UIKit] Add attr missing from the review in https://github.com/xamarin/xamarin-macios/pull/15800 (#15835)

I asked for the attr to be added but the PR was merged without that change.

src/uikit.cs

@@ -2507,6 +2507,7 @@ namespace UIKit {
 		[Deprecated (PlatformName.TvOS, 13, 0, message: "Use a 'BGAppRefreshTask' from 'BackgroundTasks' framework.")]
 		void SetMinimumBackgroundFetchInterval (double minimumBackgroundFetchInterval);
 
+		[EditorBrowsable (EditorBrowsableState.Advanced)]
 		[iOS (7,0)]
 		[Export ("preferredContentSizeCategory")]
 		NSString PreferredContentSizeCategory { get; }