[Mailer] Update .env for "send grid" recipe change

[weaverryan]

In https://symfony.com/doc/4.4/mailer.html#using-a-3rd-party-transport

It says the .env file will look like this after installing sendgrid transport:

# .env
SENDGRID_KEY=
MAILER_DSN=sendgrid://$SENDGRID_KEY@default

(and that you only need to then configure SENDGRID_KEY). That's not true anymore. The recipe no longer uses 2 env variables: https://github.com/symfony/recipes/blob/master/symfony/sendgrid-mailer/4.4/manifest.json