Correct a bug in the nested() example

pep-0343.txt

@@ -956,7 +956,10 @@ Examples
                      else:
                          exc = (None, None, None)
                  if exc != (None, None, None):
-                     raise
+                     # sys.exc_info() may have been
+                     # changed by one of the exit methods
+                     # so provide explicit exception info
+                     raise exc[0], exc[1], exc[2]
 
          Sample usage: