Update README.md

Author: RobinLinus

README.md

@@ -13,7 +13,7 @@ BitVM is the foundational element to bridge BTC to second layers such as sidecha
 
 
 ## Resources
-- [BitVM3-RSA](bitvm3-rsa.pdf)
+- [BitVM3-RSA (broken)](bitvm3-rsa.pdf)
 - [BitVM Bridge](https://bitvm.org/bitvm_bridge.pdf)
 - [BitVM 2 writeup](./bitvm2)
 - [BitVM whitepaper](bitvm.pdf)