contrib/fast-import: use a lowercase "usage:" string

Make the usage string consistent with Git.

Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/contrib/fast-import/git-import.perl b/contrib/fast-import/git-import.perl
index f9fef6d..0891b9e 100755
--- a/contrib/fast-import/git-import.perl
+++ b/contrib/fast-import/git-import.perl
@@ -7,7 +7,7 @@
 use strict;
 use File::Find;
 
-my $USAGE = 'Usage: git-import branch import-message';
+my $USAGE = 'usage: git-import branch import-message';
 my $branch = shift or die "$USAGE\n";
 my $message = shift or die "$USAGE\n";