docs: improve sample code

If a dev copy and paste the constructor, it won't work
becaseu `parent::__construct();` is missing.

Author: kenjis

user_guide_src/source/database/configuration/008.php

@@ -4,14 +4,20 @@
 
 use CodeIgniter\Database\Config;
 
+/**
+ * Database Configuration
+ */
 class Database extends Config
 {
+    // ...
     public $development = [/* ... */];
     public $test        = [/* ... */];
     public $production  = [/* ... */];
 
     public function __construct()
     {
+        // ...
+
         $this->defaultGroup = ENVIRONMENT;
     }
 }