Add --dev to composer require server suggestion

Current advice results with adding `"symfony/web-server-bundle"` into `require` section in composer.json.

We should advice `composer require server --dev` in order to add 
`"symfony/web-server-bundle"` into `require-dev` section in composer.json

Author: marek-pietrzak-tg

symfony/framework-bundle/3.3/post-install.txt

@@ -8,6 +8,6 @@
     3. Browse to the <comment>http://localhost:8000/</> URL.
 
        Quit the server with CTRL-C.
-       Run <comment>composer require server</> for a better web server.
+       Run <comment>composer require server --dev</> for a better web server.
 
   * <fg=blue>Read</> the documentation at <comment>https://symfony.com/doc</>