Update Book link in getting started guide (guides.md)

[osanstrong]

The "Getting Started" guide links to the textbook "Effective Computation in Physics". This change updates the link from ProQuest (book no longer hosted) to the more current I-Share.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] I have read the CONTRIBUTING document.
• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

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[osanstrong]

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