Update README.md

Removing comments regarding the process.env.IS_OFFLINE variable as it has been removed by the plugin in the v6 alpha changes. See dherault#768 (comment) for more details.

README.md

@@ -177,10 +177,6 @@ aws lambda invoke /dev/null \
   --function-name myServiceName-dev-invokedHandler
 ```
 
-## The `process.env.IS_OFFLINE` variable
-
-Will be `"true"` in your handlers and thorough the plugin.
-
 ## Token authorizers
 
 As defined in the [Serverless Documentation](https://serverless.com/framework/docs/providers/aws/events/apigateway/#setting-api-keys-for-your-rest-api) you can use API Keys as a simple authentication method.