plugin: replace point-in-time with retrieved
As discussed at
using point in time, although not completely wrong, is a little
strange. Using retrieved in the source / reference instead is more
suitable. Although it is unclear if retrieved relates to the reference
URL (if any) in the sources or the value of the claim itself, it does
not make a significant difference in practice.
Signed-off-by: Loic Dachary <email@example.com>