BUG: fixed the bug in RSS extension that surface only evolve in one dire...

a missing - sign, but took two days to locate...

compare view:

https://github.com/gaoyi/RSSExtension/compare/2be89dfac932bfae034ee43c87b6383782c1fb33…6c9dc8f8519ba93ff9ef7c220366856f7fa66e43