doc: Give the absolute file name of 'hydra.gnu.org.pub' in snippets.

* doc/guix.texi (Substitute Server Authorization): Give the absolute
file name of 'hydra.gnu.org.pub' in the snippet.

doc/guix.texi

@@ -2197,7 +2197,7 @@ make sure you checked the GPG signature of
 Then, you can run something like this:
 
 @example
-# guix archive --authorize < hydra.gnu.org.pub
+# guix archive --authorize < @var{prefix}/share/guix/hydra.gnu.org.pub
 @end example
 
 Once this is in place, the output of a command like @code{guix build}