Fix 12-Proxies-and-Reflection.md chapter set trap

[lfb]

I want to correct an error in 12-Proxies-and-Reflection.md chapter：

Original：

let proxy = new Proxy(target, {
    set(trapTarget, key, value, receiver) {

        // ignore existing properties so as not to affect them
        if (!trapTarget.hasOwnProperty(key)) {
            if (isNaN(value)) {
                throw new TypeError("Property must be a number.");
            }
        }

        // add the property
        return Reflect.set(trapTarget, key, value, receiver);
    }
});

Fix：throw new TypeError ("Property must be a number.") => Throw new TypeError ("Property value must be a number.")

let proxy = new Proxy(target, {
    set(trapTarget, key, value, receiver) {

        // ignore existing properties so as not to affect them
        if (!trapTarget.hasOwnProperty(key)) {
            if (isNaN(value)) {
                throw new TypeError("Property value must be a number.");
            }
        }

        // add the property
        return Reflect.set(trapTarget, key, value, receiver);
    }
});