doc: update FPiL entry in README

/cc @david-christiansen

I think it's progressed far enough that no "in development" annotation is necessary on this page

README.md

@@ -8,7 +8,7 @@ This is the repository for **Lean 4**, which is currently being released as mile
 - [Quick tour video](https://youtu.be/zyXtbb_eYbY)
 - [Homepage](https://leanprover.github.io)
 - [Theorem Proving Tutorial](https://leanprover.github.io/theorem_proving_in_lean4/)
-- [Functional Programming in Lean](https://leanprover.github.io/functional_programming_in_lean/) **first chapter is available!**
+- [Functional Programming in Lean](https://leanprover.github.io/functional_programming_in_lean/)
 - [Manual](https://leanprover.github.io/lean4/doc/)
 - [Release notes](RELEASES.md) starting at v4.0.0-m3
 - [Examples](https://leanprover.github.io/lean4/doc/examples.html)