Update 'README.md'

README.md

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 - [Mathstodon](https://mathstodon.xyz/about)
 
 ## Resources
-- [ZLIB](https://singlelogin.re/)
+- [ZLIB](https://www.reddit.com/r/zlibrary/wiki/index/access/)
 - [Library Genesis](http://libgen.rs/)
 - [Kindle Digital Library](https://www.amazon.co.jp/hz/mycd/digital-console/contentlist/booksAll/dateDsc?pageNumber=1)
 - [SciRate](https://scirate.com/)