[react] [readme] Fix broken documentation links and syntax error in code example #3303

Description

The React/JSX Style Guide (react/README.md) contains 3 broken external links and 1 syntax error in a code example.

Broken Links (all return HTTP 404)

All three facebook.github.io/react URLs are dead — React's docs moved to react.dev and the legacy blog content is now at legacy.reactjs.org.

Working replacements (verified):

• legacy.reactjs.org/docs/react-without-es6.html ✅ 200
• legacy.reactjs.org/blog/2016/07/13/mixins-considered-harmful.html ✅ 200
• legacy.reactjs.org/blog/2015/12/16/ismounted-antipattern.html ✅ 200

Syntax Error in Code Example (Line 457)

The "Spreading objects with known, explicit props" example has invalid JavaScript — function Foo is missing its parameter parentheses ():

// Current (SyntaxError — won't parse)
export default function Foo {
  const props = { text: '', isPublished: false }
  return (<div {...props} />);
}


// Fixed
export default function Foo() {
  const props = { text: '', isPublished: false }
  return (<div {...props} />);
}