Fix capitalisation of WhatsApp

Author: Xethron

README.md

@@ -46,10 +46,10 @@ $balance = $messageBird->balance->read();
 ```
 
 
-Conversations Whatsapp Sandbox
+Conversations WhatsApp Sandbox
 -------------
 
-To use the whatsapp sandbox you need to add `\MessageBird\Client::ENABLE_CONVERSATIONSAPI_WHATSAPP_SANDBOX` to the list of features you want enabled. Don't forget to replace `YOUR_ACCESS_KEY` with your actual access key.
+To use the WhatsApp sandbox you need to add `\MessageBird\Client::ENABLE_CONVERSATIONSAPI_WHATSAPP_SANDBOX` to the list of features you want enabled. Don't forget to replace `YOUR_ACCESS_KEY` with your actual access key.
 
 ```php
 $messageBird = new \MessageBird\Client('YOUR_ACCESS_KEY', null, [\MessageBird\Client::ENABLE_CONVERSATIONSAPI_WHATSAPP_SANDBOX]);