readme: fix link to original paper (#4391)

Signed-off-by: Marko Baricevic <marbar3778@yahoo.com>

README.md

@@ -157,6 +157,5 @@ Additional documentation is found [here](/docs/tools).
 
 - [The latest gossip on BFT consensus](https://arxiv.org/abs/1807.04938)
 - [Master's Thesis on Tendermint](https://atrium.lib.uoguelph.ca/xmlui/handle/10214/9769)
-- [Original Whitepaper](https://github.com/tendermint/spec)
-  - You can find the link at the bottom of the readme
+- [Original Whitepaper: "Tendermint: Consensus Without Mining"](https://tendermint.com/static/docs/tendermint.pdf)
 - [Blog](https://blog.cosmos.network/tendermint/home)