Fix flow in Navigation Playground (#735)

examples/NavigationPlayground/js/App.js

@@ -68,7 +68,7 @@ const ExampleRoutes = {
 const MainScreen = ({ navigation }) => (
   <ScrollView>
     <Banner />
-    {Object.keys(ExampleRoutes).map((routeName: String) =>
+    {Object.keys(ExampleRoutes).map((routeName: string) =>
       <TouchableOpacity
         key={routeName}
         onPress={() => {

examples/NavigationPlayground/js/SampleText.js

@@ -10,7 +10,13 @@ import {
 /**
  * Used across examples as a screen placeholder.
  */
-const SampleText = ({ children }) => (
+ import type { Children } from 'react';
+
+const SampleText = ({
+  children
+}: {
+  children?: Children
+}) => (
   <Text style={styles.sampleText}>{children}</Text>
 );