Update 'backlog.md'

backlog.md

@@ -59,6 +59,7 @@
 - [Dice Problem](https://blog.computationalcomplexity.org/2024/01/a-nice-dice-problem-part-2.html)
 - [Polynomials and RE](https://blog.computationalcomplexity.org/2024/01/certifying-number-is-in-set-using.html)
 - [Holevo's Theorem](https://en.wikipedia.org/wiki/Holevo%27s_theorem)
+- [Formula for Primes](https://en.wikipedia.org/wiki/Formula_for_primes)
 
 
 ## Reference Book Backlog