update .env.example template for clarity (#430)

Author: jiachen247

.env.example

@@ -1,4 +1,4 @@
 REACT_APP_IVLE_KEY=your_ivle_key_here
 REACT_APP_VERSION=$npm_package_version
-REACT_APP_BACKEND_URL=http://something.com
-REACT_APP_USE_BACKEND=
+REACT_APP_BACKEND_URL=http://localhost:4001
+REACT_APP_USE_BACKEND=TRUE

src/utils/constants.ts

@@ -5,7 +5,9 @@ dotenv.config()
 export const IVLE_KEY = process.env.REACT_APP_IVLE_KEY
 export const VERSION = process.env.REACT_APP_VERSION
 export const BACKEND_URL = process.env.REACT_APP_BACKEND_URL
-export const USE_BACKEND = process.env.REACT_APP_USE_BACKEND
+export const USE_BACKEND =
+  process.env.REACT_APP_USE_BACKEND !== '' &&
+  process.env.REACT_APP_USE_BACKEND!.toUpperCase() === 'TRUE'
 
 export enum LINKS {
   GITHUB_ISSUES = 'https://github.com/source-academy/cadet-frontend/issues',