Use less misleading default value for --http-headers

src/Command.php

@@ -42,7 +42,7 @@ protected function configure(): void
                 null,
                 InputOption::VALUE_REQUIRED,
                 'Custom HTTP headers to append to response (in JSON format). Eg. --http-headers=\'["Content-encoding: gzip"]\'',
-                '{}'
+                '[]'
             )
             ->setDescription('Turn any STDIN/STDOUT into HTTP server')
         ;