Fix C# code example for Object._get_property_list

There were two small errors in this code example that kept it from working when copied to a new node locally. These are the fixes I used locally to test the example.

doc/classes/Object.xml

@@ -136,7 +136,7 @@
 				        }
 				    }
 
-				    private List<int> _numbers = new();
+				    private Godot.Collections.Array<int> _numbers = new();
 
 				    public override Godot.Collections.Array<Godot.Collections.Dictionary> _GetPropertyList()
 				    {
@@ -173,7 +173,7 @@
 				        if (propertyName.StartsWith("number_"))
 				        {
 				            int index = int.Parse(propertyName.Substring("number_".Length));
-				            numbers[index] = value.As<int>();
+				            _numbers[index] = value.As<int>();
 				            return true;
 				        }
 				        return false;