correct '-dbcache' to '-prune'

Author: BenWestgate

doc/design/assumeutxo.md

@@ -16,7 +16,7 @@ load it.
 A pruned node can load a snapshot. To save space, it's possible to delete the
 snapshot file as soon as `loadtxoutset` finishes.
 
-The minimum `-dbcache` setting is 550 MiB, but this functionality ignores that
+The minimum `-prune` setting is 550 MiB, but this functionality ignores that
 minimum and uses at least 1100 MiB.
 
 As the background sync continues there will be temporarily two chainstate