Improve the class component defaultProps example

In the current example, we have a type definition for props but do not
make use of it for defaultProps. This can lead to a mismatch between
defaultProps and props.

README.md

@@ -445,7 +445,7 @@ export class Greet extends React.Component<Props> {
     const { name } = this.props;
     return <div>Hello ${name.toUpperCase()}!</div>;
   }
-  static defaultProps = { name: 'world' };
+  static defaultProps: Partial<Props> = { name: 'world' };
 }
 
 // Type-checks! No type assertions needed!