docs: fix access to env var using VITE_ prefix in README.md

[dandv]

Priority*

• High: This PR needs to be merged first, before other tasks.
• Medium: This PR should be merged quickly to prevent conflicts due to common changes. (default)
• Low: This PR does not affect other tasks, so it can be merged later.

Purpose of the PR*

In my previous large PR I forgot to add the VITE_ prefix to accessing the env var via import.meta.env.

Reference

https://vite.dev/guide/env-and-mode#env-files

[dandv]

I haven't tested this super thoroughly, but it seems that adding an env var to .env requires restarting pnpm dev for the variable to be accessible via import.meta.env.VITE.... If that is indeed the case, it should be mentioned in that section.

[Jonghakseo]

LGTM