Update MarkdownImageHandler.swift

[nicoster]

so I can create my own MarkdownImageHandler, like the following. without the change, It complains 'MarkdownImageHandler' initializer is inaccessible due to 'internal' protection level

let myNetworkImage = MarkdownImageHandler(imageAttachment: { url in

    return Deferred {
        Future<NSTextAttachment, Never> { promise in
            KingfisherManager.shared.retrieveImage(with: ImageResource(downloadURL: url), completionHandler: { result in
                switch result {
                case .success(let value):
                    let attachment = NSTextAttachment()
                    attachment.image = value.image
                    promise(.success(attachment))
                case .failure(let error):
                    promise(.success(NSTextAttachment()))
                }
            })
        }
    }.eraseToAnyPublisher()
})