Adding wordpress environment variable.

Wordpress needs to know what db to it's user has access to. Without this mysql throws access denied errors.

compose/wordpress.md

@@ -59,6 +59,7 @@ Compose to set up and run WordPress. Before starting, make sure you have
            WORDPRESS_DB_HOST: db:3306
            WORDPRESS_DB_USER: wordpress
            WORDPRESS_DB_PASSWORD: wordpress
+           WORDPRESS_DB_NAME: wordpress
     volumes:
         db_data: {}
     ```