Fix customObject sample (rjsf-team#716)

The sample for customObject.js had few defects. React 15.6.1 throws an no-keys-defined warning as well as 'child-can-not-be-objects' exception. The code has been changed to fix these issues. * Fixed customObject sample to fix "Objects are not valid as a React child" error for react 15.6.1 and to avoid "https://fb.me/react-warning-keys" warning * Run prettier

Author: Danil Zhelnov

playground/samples/customObject.js

@@ -6,7 +6,11 @@ function ObjectFieldTemplate({ TitleField, properties, title, description }) {
  <TitleField title={title} />
  <div className="row">
  {properties.map(prop => (
- <div className="col-lg-2 col-md-4 col-sm-6 col-xs-12">{prop}</div>
+ <div
+ className="col-lg-2 col-md-4 col-sm-6 col-xs-12"
+ key={prop.content.key}>
+ {prop.content}
+ </div>
  ))}
  </div>
  {description}