Remove irrelevant reference from "Tying it all together"

Sorry Jon, but this might not be of any help to new Git users ;)

Acked-by: Jon Loeliger <jdl@jdl.com>
Signed-off-by: Thomas Ackermann <th.acker@arcor.de>
"git prune" is safe

"git prune" is safe in case of concurrent accesses to a repository
but using it in such a case is not recommended.

Signed-off-by: Thomas Ackermann <th.acker@arcor.de>