[scipy.md] Update np.random → Generator API

[Chihiro2000GitHub]

Summary

This PR updates lectures/scipy.md as part of the NumPy random API migration described in QuantEcon/meta#299.

The changes are limited to the following:

• Replace np.random.beta(...) with rng = np.random.default_rng() / rng.beta(...)
• Update the surrounding wording: "functions" → "tools" to avoid implying module-level functions
• Replace np.random.randn(...) with rng.standard_normal(...) in "12.3.3. Other Goodies in scipy.stats"
• Replace np.random.randn(...) with rng.standard_normal(...) in the Monte Carlo exercise solution (Exercise 12.3)

Each code block defines its own rng locally. The exercise solution block remains self-contained.
No fixed seed is introduced, as the original lecture did not use one.
No Numba-related code was present in this file.

Hi @mmcky and @HumphreyYang, I'd be grateful if you could take a look when you have time.

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