Fix Pauli-basis expansion normalization

[matti-sarjala]

What changed

This corrects the Pauli-basis expansion formula in docs/guides/specify-observables-pauli.ipynb by adding the missing 1/2^n Hilbert--Schmidt normalization factor.

Why

For standard n-qubit Pauli strings,

\mathrm{Tr}(P_i P_j) = 2^n \delta_{ij},

so the coefficient of a Pauli string P in the expansion of an observable O is

\alpha_P = \frac{1}{2^n}\mathrm{Tr}(O P).

This also matches the coefficients printed by the example immediately below the formula.

Risk

Documentation-only change. No code or API behavior is changed.

Tested

Checked the formula against the 2-qubit matrix example in the same guide:
Tr(O IZ) = -4, Tr(O XI) = 2, and Tr(O YY) = 4, which become -1, 0.5, and 1 after division by 2^2 = 4.

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