Fix grammar, spacing, and code reference issues in README.md

[juni2003]

This PR addresses several documentation issues:

1. Grammar fixes:

   • Line 40 & 81: Added missing article "the" in "Why is the sky blue?"
   • Line 68: Removed redundant period after question mark in "What things do I like?"
   • Line 87: Removed extra word "is" in "tools that the model can use"
   • Line 88: Corrected possessive "its" (was incorrectly "it's") in "specifying its schema"

2. Spacing fixes:

   • Line 187: Added space before parenthesis in "bring-your-own-response (BYOR)"

3. Code consistency fix:

   • Lines 57: Changed unqualified reference to Part class to use consistent generative_models.Part qualifier, matching line 53

These changes improve documentation readability and prevent potential errors in example code.

Note: This PR contains small documentation and example code fixes only, no functional changes.

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