worktree: fix option descriptions for `prune`

The `verbose` and `expire` options of the `git worktree prune`
subcommand have wrong descriptions in that they pretend to relate to
objects. But as the git-worktree(1) correctly states, these options have
nothing to do with objects but only with worktrees. Fix the description
accordingly.

Signed-off-by: Patrick Steinhardt <patrick.steinhardt@elego.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/worktree.c

@@ -105,9 +105,9 @@ static int prune(int ac, const char **av, const char *prefix)
 {
 	struct option options[] = {
 		OPT__DRY_RUN(&show_only, N_("do not remove, show only")),
-		OPT__VERBOSE(&verbose, N_("report pruned objects")),
+		OPT__VERBOSE(&verbose, N_("report pruned working trees")),
 		OPT_EXPIRY_DATE(0, "expire", &expire,
-				N_("expire objects older than <time>")),
+				N_("expire working trees older than <time>")),
 		OPT_END()
 	};