update composer ubuntu 20.04 code example

Example: install composer on ubuntu 20.04

# download composer.phar to /usr/bin and symlink composer to composer.phar
pushd /usr/bin >/dev/null &&
sudo wget -O composer.phar https://getcomposer.org/composer-stable.phar &&
sudo chmod 755 composer.phar &&
sudo ln -fs composer.phar composer &&
popd >/dev/null