Fix typos in example descriptions.

Summary: Closes https://github.com/facebook/react-native/pull/3095

Reviewed By: svcscm

Differential Revision: D2559729

Pulled By: foghina

fb-gh-sync-id: 70dbd39de86e6377d32d1628c77ff10510ac3ae5

Examples/UIExplorer/ToastAndroidExample.android.js

@@ -31,7 +31,7 @@ var ToastExample = React.createClass({
 
   statics: {
     title: 'Toast Example',
-    description: 'Example that demostrates the use of an Android Toast to provide feedback.',
+    description: 'Example that demonstrates the use of an Android Toast to provide feedback.',
   },
 
   getInitialState: function() {

Examples/UIExplorer/XHRExample.android.js

@@ -262,7 +262,7 @@ class FormUploader extends React.Component {
 
 exports.framework = 'React';
 exports.title = 'XMLHttpRequest';
-exports.description = 'Example that demostrates upload and download requests ' +
+exports.description = 'Example that demonstrates upload and download requests ' +
   'using XMLHttpRequest.';
 exports.examples = [{
   title: 'File Download',