Fix typo in Frontier.iter() rtype

Author: chekunkov

scrapinghub/client/frontiers.py

@@ -174,7 +174,7 @@ def iter(self):
         """Iterate through slots.
 
         :return: an iterator over frontier slots names.
-        :rtype: :class:`collections.Iterate[str]`
+        :rtype: :class:`collections.Iterable[str]`
         """
         return iter(self.list())