Fix link to Nitrokey Storage device in README

The Nitrokey team changed the URL where details about the Nitrokey
Storage device are to be found, rendering our link broken. This change
updates the link to reference the new location.

Author: d-e-s-o

README.md

@@ -156,7 +156,7 @@ the full text of the license.
 [`libnitrokey`]: https://github.com/nitrokey/libnitrokey
 [nitrokey-ug]: https://www.nitrokey.com
 [nitrokey-pro]: https://shop.nitrokey.com/shop/product/nitrokey-pro-2-3
-[nitrokey-storage]: https://shop.nitrokey.com/shop/product/nitrokey-storage-2-16gb-23
+[nitrokey-storage]: https://shop.nitrokey.com/shop/product/nitrokey-storage-2-56
 [nitrocli-arch]: https://aur.archlinux.org/packages/nitrocli
 [nitrocli-cratesio]: https://crates.io/crates/nitrocli
 [nitrocli-debian]: https://packages.debian.org/stable/nitrocli