lguest: document --rng in example Launcher

Rusty Russell wrote:
> Ah, it will appear as /dev/hwrng.  It's a weirdness of Linux that our actual
> hardware number generators are not wired up to /dev/random...

Reflected this in the documentation, thanks :-)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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