Add year to AMP reference

Author: mcbal

README.md

@@ -110,7 +110,7 @@ On deep equilibrium models:
 - [Chapter 4: Deep Equilibrium Models](https://implicit-layers-tutorial.org/deep_equilibrium_models/) of the [Deep Implicit Layers - Neural ODEs, Deep Equilibrium Models, and Beyond](http://implicit-layers-tutorial.org/), created by Zico Kolter, David Duvenaud, and Matt Johnson
 
 On approximate message passing (AMP) methods in statistics: 
-- [A unifying tutorial on Approximate Message Passing](https://arxiv.org/abs/2105.02180) by Oliver Y. Feng, Ramji Venkataramanan, Cynthia Rush, Richard J. Samworth: the example on page 2 basically describes how transformers implement approximate message passing: an iterative algorithm with a "denoising" step (attention) followed by a "memory term" or Onsager correction term (feed-forward layer)
+- [A unifying tutorial on Approximate Message Passing](https://arxiv.org/abs/2105.02180) (2021) by Oliver Y. Feng, Ramji Venkataramanan, Cynthia Rush, Richard J. Samworth: the example on page 2 basically describes how transformers implement approximate message passing: an iterative algorithm with a "denoising" step (attention) followed by a "memory term" or Onsager correction term (feed-forward layer)
 
 
 ### Code inspiration