Fix the docstring of _default_security_group

Since I3879ae24e47590d12fb09a4bb4697f139f57caf7,
DeletableSecurityGroup has been removed and the return value of
_default_security_group() has been changed.
So this patch fixes the corresponding docstring.

Change-Id: I45ec199eb80b5687d38d1a1b0f561277f8e3b37c
diff --git a/tempest/scenario/manager.py b/tempest/scenario/manager.py
index 233d747..a88194c 100644
--- a/tempest/scenario/manager.py
+++ b/tempest/scenario/manager.py
@@ -977,7 +977,7 @@
     def _default_security_group(self, client=None, tenant_id=None):
         """Get default secgroup for given tenant_id.
 
-        :returns: DeletableSecurityGroup -- default secgroup for given tenant
+        :returns: default secgroup for given tenant
         """
         if client is None:
             client = self.security_groups_client