Fix ValueError information in obj/expirer

I fix error information in raise ValueError(...)
For example:
    if a>=b:
        # It should be under below and not 'a must be less than or equal to b'
        raise ValueError('a must be less than b')

Change-Id: I3d12b79470d122b2114f9ee486b15d381f290f95

swift/obj/expirer.py

@@ -252,7 +252,7 @@ class ObjectExpirer(Daemon):
 
         if self.processes and self.process >= self.processes:
             raise ValueError(
-                'process must be less than or equal to processes')
+                'process must be less than processes')
 
     def delete_object(self, actual_obj, timestamp, container, obj):
         start_time = time()