fix(readme) switch AppSync.js to aws-exports.js (#373)

I can't find anywhere in the console that I can download a file named AppSync.js... but I have aws-exports.js.

The docs are still very confusing when new to all of these libraries that can be mixed and matched.

I can get the Amplify CLI to work out of the box, but as soon as you try to add AWSAppSyncClient instead and use the Apollo client things get very confusing to me.

tutorials/react-offline-realtime-todos/README.md

@@ -52,7 +52,7 @@ input UpdateTodoInput {
 }
 ```
 
-**Save** the schema again. On the navigation bar in the left of the console, click on `<your api name>`, scroll down and select the **Web** section then click **Download** and save the `AppSync.js` file somewhere for later.
+**Save** the schema again. On the navigation bar in the left of the console, click on `<your api name>`, scroll down and select the **Web** section then click **Download Config** and save the `aws-exports.js` file somewhere for later.
 
 ## Imports and configuration
 
@@ -65,25 +65,25 @@ create-react-app todos && cd ./todos
 yarn add aws-appsync aws-appsync-react graphql-tag react-apollo
 ```
 
-Copy the `AppSync.js` file that you downloaded from the console into the `./todos/src` directory. Next add the following imports towards the top of the `App.js` file:
+Copy the `aws-exports.js` file that you downloaded from the console into the `./todos/src` directory. Next add the following imports towards the top of the `App.js` file:
 
 
 ```javascript
 import AWSAppSyncClient, { buildSubscription } from 'aws-appsync';
 import { Rehydrated, graphqlMutation } from 'aws-appsync-react';
-import AppSyncConfig from './AppSync';
+import awsmobile from './aws-exports';
 import { ApolloProvider } from 'react-apollo';
 ```
 
 Replace everything __after__ the definition of the `<App />` component with the following configuration:
 
 ```jsx
 const client = new AWSAppSyncClient({
-  url: AppSyncConfig.graphqlEndpoint,
-  region: AppSyncConfig.region,
+  url: awsmobile.graphqlEndpoint,
+  region: awsmobile.region,
   auth: {
-    type: AppSyncConfig.authenticationType,
-    apiKey: AppSyncConfig.apiKey
+    type: awsmobile.authenticationType,
+    apiKey: awsmobile.apiKey
   }
 })