Fix scipy exercise hints

[ken-r]

Two exercise hints in the scipy lecture are not fully accurate.
This PR fixes the hints.

[mmcky]

Thanks for your PR @ken-r -- much appreciated.

[Copilot]

Pull Request Overview

This PR fixes two inaccurate exercise hints in the scipy lecture by correcting method calls and module imports.

• Corrects the lognorm usage to include the .pdf() method call
• Fixes the incorrect module reference from scipy.optimize to scipy.integrate for the quad function

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[mmcky]

Thanks again @ken-r really appreciate the feedback.

Merging now and will update the live site soon.

[ken-r]

Thank you very much for all your work and the great lecture @mmcky

[jstac]

Many thanks @ken-r, much appreciated.