docs: correct preview.js example in README

[JeffGuKang]

Issue:

preview.js makes error in README.
addon-ondevice-backgrounds needs values option.

Error

 ERROR  TypeError: undefined is not a function

This error is located at:
    in BackgroundPanel (created by Wrapper)
    in RCTScrollContentView (created by ScrollView)
    in RCTScrollView (created by ScrollView)
    in ScrollView (created by ScrollView)
    in ScrollView (created by Wrapper)
...

It looks values is missed in README.

node_modules/@storybook/addon-ondevice-backgrounds/dist/BackgroundPanel.js

53~57

    return (react_1.default.createElement(react_native_1.View, null, backgrounds ? (backgrounds.values.map(function (_a) {
        var value = _a.value, name = _a.name;
        return (react_1.default.createElement(react_native_1.View, { key: "".concat(name, " ").concat(value) },
            react_1.default.createElement(Swatch_1.default, { value: value, name: name, setBackground: setBackgroundFromSwatch })));
    })) : (react_1.default.createElement(Instructions, null))));

What I did

Correct README

How to test

Please explain how to test your changes and consider the following questions

• Does this need a new example in examples/native?
  No. Example has right option.
• Does this need an update to the documentation?
  I did.

If your answer is yes to any of these, please make sure to include it in your PR.

[dannyhw]

nice catch!

[dannyhw]

thanks for your contribution 🙇