Update README.md

update twocheckout -> omnipay/2checkout

Author: greydnls

README.md

@@ -102,7 +102,7 @@ The following gateways are available:
 
 Gateway | Composer Package | Maintainer
 --- | --- | ---
-[2Checkout](https://github.com/thephpleague/omnipay-2checkout) | omnipay/twocheckout | [Kayla Daniels](https://github.com/kayladnls)
+[2Checkout](https://github.com/thephpleague/omnipay-2checkout) | omnipay/2checkout | [Kayla Daniels](https://github.com/kayladnls)
 [Alipay](https://github.com/lokielse/omnipay-alipay) | lokielse/omnipay-alipay | [Loki Else](https://github.com/lokielse)
 [Authorize.Net](https://github.com/thephpleague/omnipay-authorizenet) | omnipay/authorizenet | [Kayla Daniels](https://github.com/kayladnls)
 [Barclays ePDQ](https://github.com/samvaughton/omnipay-barclays-epdq) | samvaughton/omnipay-barclays-epdq | [Sam Vaughton](https://github.com/samvaughton)