Fixes outdated command

`composer property-set minimum-stability "alpha"`

```
$ composer property-set minimum-stability "alpha"                                                             
                                                                                                                                                
  [Symfony\Component\Console\Exception\CommandNotFoundException]                                                                                
  Command "property-set" is not defined.                                                                                                        
                                                                                                                                                
                                           
```

`composer config minimum-stability "alpha"` did the job.

Author: gruz

README.md

@@ -35,7 +35,7 @@ composer require joomla/coding-standards "~3.0"
 As stability of joomla/coding-standards 3.0.0 is currently alpha, make sure you allow usage of alpha software in Composer:
 
 ```bash
-composer property-set minimum-stability "alpha"
+composer config minimum-stability "alpha"
 ```
 
 If you want to install the coding-standards globally, edit your Composer global configuration by running: