Style + Clarity Edits to Introduction to Ray AIR

[emmyscode]

Changes summary

Summary of changes in this PR

Top to bottom copy edits for style, clarity, correctness, and concision.

PR scope

• [ ] new notebook
• [x] improvements to the existing notebook
• [ ] bug fixes
• [ ] other

Checklist

• [x] notebook is without outputs
• [x] empty cells are removed
• [x] code is formatted with black
• [x] imports are sorted
• [ ] if new notebook is added, README is updated