doc: Remove stale reference to 'hydra.gnu.org'.

Reported by Thomas Schmitt <scdbackup@gmx.net>.

* doc/guix.texi (Binary Installation): Refer to SUBSTITUTE-SERVER, not
to 'hydra.gnu.org'.

doc/guix.texi

@@ -585,12 +585,12 @@ Info search path.)
 
 @item
 @cindex substitutes, authorization thereof
-To use substitutes from @code{hydra.gnu.org} or one of its mirrors
+To use substitutes from @code{@value{SUBSTITUTE-SERVER}} or one of its mirrors
 (@pxref{Substitutes}), authorize them:
 
 @example
 # guix archive --authorize < \
-     ~root/.config/guix/current/share/guix/hydra.gnu.org.pub
+     ~root/.config/guix/current/share/guix/@value{SUBSTITUTE-SERVER}.pub
 @end example
 
 @item

gnu/packages/package-management.scm

@@ -316,6 +316,7 @@ the Nix package manager.")
     (inputs
      `(("gnutls" ,gnutls)
        ("guile-git" ,guile-git)
+       ("guile-json" ,guile-json)
        ("guile-gcrypt" ,guile-gcrypt)
        ,@(fold alist-delete (package-inputs guix)
                '("boot-guile" "boot-guile/i686" "util-linux"))))