Update README.md: correct import

The import didn't work and confused users trying to use it.

Maybe we should also add the other import with credentials stored elsewhere.

README.md

@@ -14,7 +14,7 @@ pip install .
 
 ## Usage
 ```python
-from lofar_tmss_client import TMSSsession
+from lofar_tmss_client.tmss_http_rest_client import TMSSsession
 
 with TMSSsession(username='username', password='password', host='tmss.lofar.eu', port=443) as session:
     session.get_subtask(42)