Update configure-aws-amplify.md (#756)

Fill in the missing word to agree with following 'are'.

_chapters/configure-aws-amplify.md

@@ -50,7 +50,7 @@ const config = {
 export default config;
 ```
 
-Here we are loading the environment that are set from our serverless backend. We did this back when we were first [setting up our React app]({% link _chapters/create-a-new-reactjs-app.md %}).
+Here we are loading the environment variables that are set from our serverless backend. We did this back when we were first [setting up our React app]({% link _chapters/create-a-new-reactjs-app.md %}).
 
 ### Add AWS Amplify