Fix comment.

Fix spelling, and `VectorView` is `IReadOnlyList` in C#.

src/Avalonia.Controls/SelectionModel.cs

@@ -227,7 +227,7 @@ namespace Avalonia.Controls
                     }
 
                     // Instead of creating a dumb vector that takes up the space for all the selected items,
-                    // we create a custom VectorView implimentation that calls back using a delegate to find 
+                    // we create a custom IReadOnlyList implementation that calls back using a delegate to find 
                     // the selected item at a particular index. This avoid having to create the storage and copying
                     // needed in a dumb vector. This also allows us to expose a tree of selected nodes into an 
                     // easier to consume flat vector view of objects.