doc(AlgebraicIndependent): remove outdated TODO and add new (#9396)

The TODO item was already completed as [IsTranscendenceBasis.isAlgebraic](https://leanprover-community.github.io/mathlib4_docs/Mathlib/RingTheory/AlgebraicIndependent.html#IsTranscendenceBasis.isAlgebraic). However, transcendence degree is nowhere to be found even though it appears in the tags.



Co-authored-by: Junyan Xu <junyanxu.math@gmail.com>
Co-authored-by: Chris Hughes <chrishughes24@gmail.com>

Mathlib/RingTheory/AlgebraicIndependent.lean

@@ -31,7 +31,8 @@ This file defines algebraic independence of a family of element of an `R` algebr
 * [Stacks: Transcendence](https://stacks.math.columbia.edu/tag/030D)
 
 ## TODO
-Prove that a ring is an algebraic extension of the subalgebra generated by a transcendence basis.
+Define the transcendence degree and show it is independent of the choice of a
+transcendence basis.
 
 ## Tags
 transcendence basis, transcendence degree, transcendence