docs: broken link (#17443)

README.md

@@ -30,7 +30,7 @@ For more pointers, see [Learning Lean](https://leanprover-community.github.io/le
 ## Documentation
 
 Besides the installation guides above and [Lean's general
-documentation](https://leanprover.github.io/documentation/), the documentation
+documentation](https://docs.lean-lang.org/lean4/doc/), the documentation
 of mathlib consists of:
 
 - [The mathlib4 docs](https://leanprover-community.github.io/mathlib4_docs/index.html): documentation [generated