[fix] Picker prop types

Fix #879

packages/react-native-web/src/exports/Picker/index.js

@@ -23,7 +23,7 @@ import { arrayOf, bool, func, number, oneOfType, string } from 'prop-types';
 const pickerStyleType = StyleSheetPropType(PickerStylePropTypes);
 
 type Props = {
-  children?: Array<typeof PickerItem>,
+  children?: PickerItem | Array<typeof PickerItem>,
   enabled?: boolean,
   onValueChange?: Function,
   selectedValue?: number | string,
@@ -37,7 +37,7 @@ type Props = {
 
 class Picker extends Component<Props> {
   static propTypes = {
-    children: arrayOf(PickerItemPropType),
+    children: oneOfType([PickerItemPropType, arrayOf(PickerItemPropType)]),
     enabled: bool,
     onValueChange: func,
     selectedValue: oneOfType([number, string]),