[bug#70151] [PATCH] doc: Correct the "guix shell --container" example.

[Rostislav Svoboda <rostislav.svoboda@gmail.com>]

> Shell injection says "/run/user/$USER/gdm/Xauthority -- oops that
> shouldn't happen".

??? Shell injection? Which, what, where? What do you mean?

Without the `--preserve='^XAUTHORITY$' --expose=$XAUTHORITY` (both
needed) the Chromium doesn't start, i.e. the example doesn't work:

$ guix shell --container --network --no-cwd ungoogled-chromium
--preserve='^DISPLAY$' -- chromium
[1:12:0405/094428.353734:ERROR:bus.cc(399)] Failed to connect to the
bus: Failed to connect to socket /var/run/dbus/system_bus_socket: No
such file or directory
Authorization required, but no authorization protocol specified

[1:1:0405/094428.361802:ERROR:ozone_platform_x11.cc(239)] Missing X
server or $DISPLAY
[1:1:0405/094428.361812:ERROR:env.cc(255)] The platform failed to
initialize.  Exiting.

Cheers