Update forms.md (#8136)

The component Form in "Uncontrolled Radio Button" example doesn't need state, because it does not used in render() method.

docs/forms.md

@@ -397,17 +397,17 @@ ReactDOM.render(
 class Form extends React.Component {
   constructor(props) {
     super(props);
-    this.state = {value: 'B'};
+    this.value = 'B';
     this.handleChange = this.handleChange.bind(this);
     this.handleSubmit = this.handleSubmit.bind(this);
   }
 
   handleChange(event) {
-    this.setState({value: event.target.value});
+    this.value = event.target.value;
   }
 
   handleSubmit(event) {
-    alert('Radio button value is: ' + this.state.value);
+    alert('Radio button value is: ' + this.value);
   }
 
   render() {