Fix quickstart documentation errors

[mattapperson]

Summary

• Fixed incorrect API method calls from client.chat.completions.create to client.chat.send throughout documentation
• Fixed syntax and variable errors in README.md streaming example

Changes

OVERVIEW.md

• Updated 5 instances of client.chat.completions.create to client.chat.send to match actual SDK API

README.md

• Added missing comma after model parameter (syntax error)
• Fixed variable name from stream to result in streaming example
• Fixed typo from hunk to chunk in console.log statement

Test plan

• Verified all code examples use correct API method client.chat.send
• Verified streaming example has correct variable names and syntax
• Confirmed actual SDK implementation uses chat.send() method
• Checked all other example files are already correct