Update botmanager version in composer string

botmanager version 1.8 causes an error in composer, stating that it can't find this version. changing it to 2.0.0 lets composert install it normally.

Author: Hitmare

README.md

@@ -22,7 +22,7 @@ Installation and usage is pretty straight forward:
 ### Require this package with [Composer]
 
 ```bash
-composer require php-telegram-bot/telegram-bot-manager:^1.8
+composer require php-telegram-bot/telegram-bot-manager:^2.0.0
 ```
 
 **NOTE:** This will automatically also install [PHP Telegram Bot][github-tgbot-core] into your project (if it isn't already).
@@ -33,7 +33,7 @@ It is possible however, to override the core version that this library requires:
 
 ```yaml
 "require": {
-    "php-telegram-bot/telegram-bot-manager": "^1.8",
+    "php-telegram-bot/telegram-bot-manager": "^2.0.0",
     "longman/telegram-bot": "dev-master as 0.78"
 }
 ```