Update "start" to auto-reload on WSL

I'm developing on the Windows subsystem for Linux,
and the dev server wasn't auto-reloading when I
made changes to the site. Followed the advice here
to get a fix:

facebook/create-react-app#10253

package.json

@@ -15,7 +15,7 @@
     "web-vitals": "^1.1.2"
   },
   "scripts": {
-    "start": "react-scripts start",
+    "start": "CHOKIDAR_USEPOLLING=true react-scripts start",
     "build": "react-scripts build",
     "test": "react-scripts test",
     "eject": "react-scripts eject",