sched: SchedAssert: fix type documentation for the level parameter

The type of the level parameter for getResidency(), assertResidency()
and assertSwitch() is not displayed in the HTML documentation.
trappy.stats.Topology expects a string, so report that as the type.
diff --git a/bart/sched/SchedAssert.py b/bart/sched/SchedAssert.py
index d716c96..dd0b427 100755
--- a/bart/sched/SchedAssert.py
+++ b/bart/sched/SchedAssert.py
@@ -149,7 +149,7 @@
         of the task
 
         :param level: The topological level to which the group belongs
-        :type level (hashable):
+        :type level: str
 
         :param node: The group of CPUs for which residency
             needs to calculated
@@ -195,7 +195,7 @@
             percent=False):
         """
         :param level: The topological level to which the group belongs
-        :type level (hashable):
+        :type level: str
 
         :param node: The group of CPUs for which residency
             needs to calculated
@@ -324,7 +324,7 @@
            :code:`from_node` to the :code:`to_node`:
 
         :param level: The topological level to which the group belongs
-        :type level (hashable):
+        :type level: str
 
         :param from_node: The node from which the task switches out
         :type from_node: list