docs: Fix parameter type and return value to match trait signature

[0xminds]

I noticed a mismatch in the parameter type and return value compared to the trait signature. I've updated the parameter b: felt252 to t: T and corrected the return value to (T, felt252) to align with the expected trait implementation.

This should resolve the inconsistency.

[reviewable-StarkWare]

This change is 

[orizi]

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @0xminds)

---

adrs/003_simple_trait_system.md line 30 at r1 (raw file):

impl MyImpl<A> for MyTrait<A, felt252> {
  type T = A;
  fn f(a: A, t: T) -> (T, felt252) { ... }

Suggestion:

trait MyTrait<A, B> {
  type T;
  fn f(a: A, t: Self::T) -> (Self::T, B);
}

This item introduces an "interface", with function signatures and associated types that need to be
defined by the implementations.
Note that there is no Self or a type this trait is for. It is stand alone.

Impl top level item

impl MyImpl<A> for MyTrait<A, felt252> {
  type T = A;
  fn f(a: A, t: A) -> (A, felt252) { ... }

[orizi]

Reviewable status: 0 of 1 files reviewed, 2 unresolved discussions (waiting on @0xminds)

---

adrs/003_simple_trait_system.md line 28 at r2 (raw file):

### Impl top level item

impl MyTrait<A, felt252> for MyImpl {

now it is just plain wrong.