doc: Improve "guix pack -f docker" example.

* doc/guix.texi (Invoking guix pack): Provide more concrete instructions
for the "guix pack -f docker" example.

doc/guix.texi

@@ -5057,12 +5057,21 @@ Alternatively, you can produce a pack in the Docker image format using
 the following command:
 
 @example
-guix pack -f docker guile emacs geiser
+guix pack -f docker -S /bin=bin guile guile-readline
 @end example
 
 @noindent
 The result is a tarball that can be passed to the @command{docker load}
-command.  See the
+command, followed by @code{docker run}:
+
+@example
+docker load < @var{file}
+docker run -ti guile-guile-readline /bin/guile
+@end example
+
+@noindent
+where @var{file} is the image returned by @var{guix pack}, and
+@code{guile-guile-readline} is its ``image tag''.  See the
 @uref{https://docs.docker.com/engine/reference/commandline/load/, Docker
 documentation} for more information.