pldb: Correct the usage for the `list' command.

diff --git a/pldb b/pldb
index b988145..631e544 100755 (executable)
--- a/pldb
+++ b/pldb
@@ -260,7 +260,7 @@ defcmd "next [-pu] [-o N] [-n N] GROUP [LIST]", sub {
   defined $prevlist and print "$prevlist\n", map "\t$_\n", @out;
 };
 
-defcmd "list LIST", sub {
+defcmd "list [-g [MARK=]GROUP,...] [-r [LO][-[HI]]] LIST", sub {
   local @ARGV = @_;
   my $bogusp = 0;
   my %opt;