Fixed assertion error message

Author: ojh

pandas/core/generic.py

@@ -2828,7 +2828,7 @@ def truncate(self, before=None, after=None, copy=True):
         if before is not None and after is not None:
             if before > after:
                 raise AssertionError('Truncate: %s must be after %s' %
-                                     (before, after))
+                                     (after, before))
 
         result = self.ix[before:after]