Change returnKeyType in AutoExpandingTextInput example

Summary:This change will change the returnKeyType to be "return" instead of being "done" since this example is trying to provide a example of a auto growing text input multiple use-case. It makes that the return key says just return in order to expand the input instead of done which does not really show the use of multiline.
Closes https://github.com/facebook/react-native/pull/6991

Differential Revision: D3184157

fb-gh-sync-id: dd0c21433009600d3cc585b941b089cab4895e7a
fbshipit-source-id: dd0c21433009600d3cc585b941b089cab4895e7a

Examples/UIExplorer/TextInputExample.ios.js

@@ -695,7 +695,7 @@ exports.examples = [
           <AutoExpandingTextInput
             placeholder="height increases with content"
             enablesReturnKeyAutomatically={true}
-            returnKeyType="done"
+            returnKeyType="default"
           />
         </View>
       );