Add explicit SPI import in docs [skip ci] (#229)

Author: davdroman

docs/SwiftUIIntrospect.md

@@ -158,6 +158,8 @@ Implement your own selector
 In case SwiftUIIntrospect doesn't support the SwiftUI element that you're looking for, you can implement your own selector. For example, to introspect a `TextField`:
 
 ```swift
+@_spi(Internals) import SwiftUIIntrospect
+
 public struct TextFieldType: IntrospectableViewType {}
 
 extension IntrospectableViewType where Self == TextFieldType {