unknown option wording tweak

Because getopt::long is used in passthrough mode, if a known
option like --wikiname that needs a parameter is specified w/o
the parameter, it will not be processed, and passed on through.
So in this case the "unknown option" message is innaccurate.
Make it slightly better by noting that the problem can be a missing
parameter.

IkiWiki.pm

@@ -533,7 +533,7 @@ sub loadplugins () {
 
 	run_hooks(getopt => sub { shift->() });
 	if (grep /^-/, @ARGV) {
-		print STDERR "Unknown option: $_\n"
+		print STDERR "Unknown option (or missing parameter): $_\n"
 			foreach grep /^-/, @ARGV;
 		usage();
 	}