t/lei-externals: favor "-o format:$PATHNAME" over "-f"

It'll be less ambiguous for inputs with "lei convert" and "lei import"

cf. https://public-inbox.org/meta/20210217044032.GA17934@dcvr/

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/PublicInbox/LeiToMail.pm b/lib/PublicInbox/LeiToMail.pm
index 8a2d9471..b90756ae 100644
--- a/lib/PublicInbox/LeiToMail.pm
+++ b/lib/PublicInbox/LeiToMail.pm
@@ -358,7 +358,7 @@ sub new {
                 require PublicInbox::MboxReader if $lei->{opt}->{augment};
                 (-d $dst || (-e _ && !-w _)) and die
                         "$dst exists and is not a writable file\n";
-                $self->can("eml2$fmt") or die "bad mbox --format=$fmt\n";
+                $self->can("eml2$fmt") or die "bad mbox format: $fmt\n";
                 $self->{base_type} = 'mbox';
         } else {
                 die "bad mail --format=$fmt\n";