Update mail host in .env.example

Mail host is configured as "mailpit" but in the docker-compose.yml file it's called "mail". 
Following the same nomenclature choice for "db" i'd say keep "mail" in docker-compose in case users want to switch the "mail" container to something else (mailtrap, mailhog, etc.)

Author: wuhei

.env.example

@@ -47,7 +47,7 @@ REDIS_PASSWORD=null
 REDIS_PORT=6379
 
 MAIL_MAILER=smtp
-MAIL_HOST=mailpit
+MAIL_HOST=mail
 MAIL_PORT=1025
 MAIL_USERNAME=null
 MAIL_PASSWORD=null