docs(): Fix an outdated command (docker-compose)

1. There are examples using `docker-compose ...`; so, this commit replaces all mentions of `docker-compose ...` with `docker compose ...` because the latest version of Docker now ships with `docker compose` instead of `docker-compose`.

2. ("your docker-compose" -> "your Compose file) While reading the official Docker documentation, I noticed that they normally call it a Compose file.

Author: soobinrho

README.md

@@ -142,12 +142,12 @@ [email protected]
 FLARUM_TITLE=Test flarum
 ```
 
-Run your docker-compose
+Run your Compose file
 
 ```sh
-docker-compose up -d mariadb
+docker compose up -d mariadb
 # Wait a moment for the creation of the database
-docker-compose up -d flarum
+docker compose up -d flarum
 ```
 
 * :warning: Your admin password must contain at least **8 characters** (FLARUM_ADMIN_PASS).