Added version number when install via Composer (laravel#7691)

Author: skys215

installation.md

@@ -157,7 +157,7 @@ If you do not specify which services you would like configured, a default stack
 
 If your computer already has PHP and Composer installed, you may create a new Laravel project by using Composer directly. After the application has been created, you may start Laravel's local development server using the Artisan CLI's `serve` command:
 
-    composer create-project laravel/laravel example-app
+    composer create-project laravel/laravel:^8.0 example-app
 
     cd example-app