Fixed console error in GitHub users example

[yuri-sakharov]

Appears error after clicking on "Creatable?" checkbox in GitHub users (Async with fetch.js) example.

The error:
app.js:323 Warning: GithubUsers is changing an uncontrolled input of type checkbox to be controlled. Input elements should not switch from uncontrolled to controlled (or vice versa). Decide between using a controlled or uncontrolled input element for the lifetime of the component. More info: https://fb.me/react-controlled-components

[coveralls]

Coverage remained the same at 94.118% when pulling 2823b81 on yuri-sakharov:fix/console-error-in-example into d2a3afd on JedWatson:master.

[coveralls]

Coverage increased (+0.008%) to 94.126% when pulling 2823b81 on yuri-sakharov:fix/console-error-in-example into d2a3afd on JedWatson:master.

[JedWatson]

Thanks for catching that @yuri-sakharov