Fix reference to inner gradient structure in docs basics.md

[AbelHo]

typo correction
grad4.inner.θ3 -> grad4[2].inner.θ3

at

julia> grad4 = gradient(|>, 5, poly4)
(1.0, (outer = (θ3 = [1.0, 17.5, 306.25],), inner = (θ3 = [0.5, 2.5, 12.5],)))

Here grad4.inner.θ3 corresponds to poly4.inner.θ3. These matching nested structures are at the core of how Flux works.

[Copilot]

Pull Request Overview

This PR fixes a typo in the documentation where the gradient access syntax was incorrect. The change corrects the reference to show the proper indexing into the gradient tuple structure.

• Corrects gradient access syntax from grad4.inner.θ3 to grad4[2].inner.θ3

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