update readme for 2.1

Author: cebe

README.md

@@ -23,7 +23,7 @@ composer require --prefer-dist yiisoft/yii2-apidoc
 The above command may not work on an existing project due to version conflicts that need to be resolved, so it is preferred to add the package manually to the require section of your composer.json:
 
 ```json
-"yiisoft/yii2-apidoc": "~2.0.0"
+"yiisoft/yii2-apidoc": "~2.1.0"
 ```
 
 afterwards run `composer update`. You may also run `composer update yiisoft/yii2-apidoc cebe/markdown` if you want to avoid updating unrelated packages.