doc: update swiftUI image extension helper in readme

[pauarmada]

On SwiftUI, we should generate the Image directly from the ImageResource. Loading svgs by making a UIImage first causes scaling issues resulting in blurry images.

Above: Image(uiImage: MR.images()[keyPath: \.ic_close].toUIImage()!)
Bottom:

extension Image {
    init(resource: KeyPath<MR.images, ImageResource>) {
        let imageResource = MR.images()[keyPath: resource]
        self.init(imageResource.assetImageName, bundle: imageResource.bundle)
    }
}