Update README.md

Author: Guillaume Badi

README.md

@@ -48,7 +48,7 @@ result = mailjet.contact.get(filters={'limit': 2})
 ## `POST` request
 ``` python
 # Register a new sender email address
-result = mailjet.sender.post(data={'email': '[email protected]'})
+result = mailjet.sender.create(data={'email': '[email protected]'})
 ```
 
 ## Combine a resource with an action