fix: Usage (#309)

Author: shunyue1320

README.md

@@ -35,20 +35,26 @@ open http://localhost:8000
 ```js | pure
 import Form, { Field } from 'rc-field-form';
 
-<Form
-  onFinish={values => {
-    console.log('Finish:', values);
-  }}
->
-  <Field name="username">
-    <Input placeholder="Username" />
-  </Field>
-  <Field name="password">
-    <Input placeholder="Password" />
-  </Field>
+const Input = ({ value = "", ...props }) => <input value={value} {...props} />;
 
-  <button>Submit</button>
-</Form>;
+const Demo = () => {
+  return (
+    <Form
+      onFinish={(values) => {
+        console.log("Finish:", values);
+      }}
+    >
+      <Field name="username">
+        <Input placeholder="Username" />
+      </Field>
+      <Field name="password">
+        <Input placeholder="Password" />
+      </Field>
+
+      <button>Submit</button>
+    </Form>
+  );
+};
 
 export default Demo;
 ```