code: func (c *Client) UpdateItemWebhook(accessToken, webhook string) (resp UpdateItemWebhookResponse, err error) {
  	if accessToken == "" || webhook == "" {
  		return resp, errors.New("/item/webhook/update - access token and webhook must be specified")
  	}
  
  	jsonBody, err := json.Marshal(updateItemWebhookRequest{
  		ClientID:    c.clientID,
  		Secret:      c.secret,
  		AccessToken: accessToken,
  		Webhook:     webhook,
  	})
  
  	if err != nil {
  		return resp, err
  	}
  
  	err = c.Call("/item/webhook/update", jsonBody, &resp)
  	return resp, err
  }
docs: // UpdateItemWebhook updates the webhook associated with an Item.
  // See https://plaid.com/docs/api/#update-webhook.