Update installation docs to latest version of composer-asset-plugin [ci skip]

Anyone stuck on 1.0.0 will be riddled with deprecation notices.

docs/guide/start-installation.md

@@ -37,7 +37,7 @@ by running `composer self-update`.
 
 With Composer installed, you can install Yii by running the following commands under a Web-accessible folder:
 
-    composer global require "fxp/composer-asset-plugin:~1.0.0"
+    composer global require "fxp/composer-asset-plugin:~1.0.3"
     composer create-project --prefer-dist yiisoft/yii2-app-basic basic
 
 The first command installs the [composer asset plugin](https://github.com/francoispluchino/composer-asset-plugin/)