Update FUICustomAuthUIDelegate.swift (#1205)

Update sample based on learnings discussed in #1203

Author: paulb777

samples/swift/FirebaseUI-demo-swift/Samples/Auth/FUICustomAuthUIDelegate.swift

@@ -20,7 +20,7 @@ import FirebaseEmailAuthUI
 
 class FUICustomAuthDelegate: NSObject, FUIAuthDelegate {
 
-  func authUI(_ authUI: FUIAuth, didSignInWith authDataResult: AuthDataResult?, error: Error?) {
+  @objc func authUI(_ authUI: FUIAuth, didSignInWith authDataResult: AuthDataResult?, error: Error?) {
     switch error {
     case .some(let error as NSError) where UInt(error.code) == FUIAuthErrorCode.userCancelledSignIn.rawValue:
       print("User cancelled sign-in")