Change start script to use npm instead of yarn (#16)

The repo readme says to use npm so it's weird to start the app with yarn.

Author: nstokoe

package.json

@@ -22,7 +22,7 @@
     "typescript": "^4.5.4"
   },
   "scripts": {
-    "start": "concurrently \"yarn start-frontend\" \"yarn start-api\"",
+    "start": "concurrently \"npm run start-frontend\" \"npm run start-api\"",
     "build": "react-scripts build",
     "test": "react-scripts test --env=jsdom",
     "eject": "react-scripts eject",
@@ -35,4 +35,4 @@
     "not ie <= 11",
     "not op_mini all"
   ]
-}
+}