mariadb: stack example to use MARIADB_ROOT_PASSWORD

Trying to use MariaDB names more so to highlight distinction
from MySQL.

mariadb/stack.yml

@@ -7,7 +7,7 @@ services:
     image: mariadb
     restart: always
     environment:
-      MYSQL_ROOT_PASSWORD: example
+      MARIADB_ROOT_PASSWORD: example
 
   adminer:
     image: adminer