Update u2f_migration.md

Update the example code to represent changes in the API of the `WebAuthn::AuthenticatorAssertionResponse` class

docs/u2f_migration.md

@@ -82,15 +82,16 @@ During authentication verification phase, you must pass either the original AppI
 
 ```ruby
 assertion_response = WebAuthn::AuthenticatorAssertionResponse.new(
- credential_id: params[:id],
+ user_handle: params[:response][:userHandle],
  authenticator_data: params[:response][:authenticatorData],
  client_data_json: params[:response][:clientDataJSON],
  signature: params[:response][:signature],
 )
 
 assertion_response.verify(
  expected_challenge,
- allowed_credentials: [credential],
+ public_key: credential.public_key,
+ sign_count: credential.count,
  rp_id: params[:clientExtensionResults][:appid] ? domain.to_s : domain.host,
 )
 ```