Aplied changes suggested by @fedemkr

Co-Authored-By: FabriBertani <fabri_bertani@hotmail.com>

Author: fedemkr

Cirrious.FluentLayout/AdvancedFluentLayoutExtensions.cs

@@ -47,7 +47,7 @@ public static FluentLayout AtBottomOf(this UIView view, UIView parentView, nfloa
 
         public static FluentLayout AtBottomOfWithSafeArea(this UIView view, UIView parentView, nfloat? margin = null) =>
             UIDevice.CurrentDevice.CheckSystemVersion(11, 0)
-                    ? view.Bottom().EqualTo().BottomOf(parentView.SafeAreaLayoutGuide).Plus(margin.GetValueOrDefault(DefaultMargin))
+                    ? view.Bottom().EqualTo().BottomOf(parentView.SafeAreaLayoutGuide).Minus(margin.GetValueOrDefault(DefaultMargin))
                     : view.AtBottomOf(parentView, margin);
 
         public static FluentLayout AtLeadingOf(this UIView view, UIView parentView, nfloat? margin = null) =>