docs(readme): correct default value for preflight option

[inyourtime]

ref:

fastify-cors/index.js

Lines 9 to 21 in 39e2d96

	const defaultOptions = {
	origin: '*',
	methods: 'GET,HEAD,POST',
	hook: 'onRequest',
	preflightContinue: false,
	optionsSuccessStatus: 204,
	credentials: false,
	exposedHeaders: null,
	allowedHeaders: null,
	maxAge: null,
	preflight: true,
	strictPreflight: true
	}

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

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