docs: update Guix package link

[FirmaSpring]

Problem

The Guix install section linked to a version-pinned page
https://guix.gnu.org/en/packages/fd-8.1.1/. That URL no longer serves the
package page; GET follows to the packages index (https://packages.guix.gnu.org/)
instead of the fd package.

The current package page is https://packages.guix.gnu.org/packages/fd/.

Verified:

GET https://guix.gnu.org/en/packages/fd-8.1.1/ -> 200, final URL is https://packages.guix.gnu.org/
GET https://packages.guix.gnu.org/packages/fd/ -> 200, stays on the fd package page

Solution

Link the unversioned Guix package page so the README does not go stale
when fd's Guix version changes.

Verification

GET https://packages.guix.gnu.org/packages/fd/ returns 200.

Documentation-only README change. I did not add a CHANGELOG entry because
this does not change fd's runtime behavior.

[tmccombs]

That link gives me a page saying that fd is an umknown package

[FirmaSpring]

The unversioned https://packages.guix.gnu.org/packages/fd/ page currently shows Unknown package, even though Guix still ships fd.

The package is still defined as fd 10.4.2 in rust-apps.scm (crate name fd-find). The versioned page https://packages.guix.gnu.org/packages/fd/10.4.2/ loads, so the README now points there. guix install fd is unchanged.