doc(Data/Finset/Defs): switch to more informative link (#39590)

After 79e17d0 `Basic.lean` does not contain any information about big operators. This PR changes the cross-link to `Defs.lean` that now contains the documentation about big oeprators.

Author: petrosagg

Mathlib/Data/Finset/Defs.lean

@@ -30,7 +30,7 @@ Finsets give a basic foundation for defining finite sums and products over types
   2. `∏ i ∈ (s : Finset α), f i`.
 
 Lean refers to these operations as big operators.
-More information can be found in `Mathlib/Algebra/BigOperators/Group/Finset/Basic.lean`.
+More information can be found in `Mathlib/Algebra/BigOperators/Group/Finset/Defs.lean`.
 
 Finsets are directly used to define fintypes in Lean.
 A `Fintype α` instance for a type `α` consists of a universal `Finset α` containing every term of