fix with-mqtt-js example. (#23636)
fix with-mqtt-js example. (#23636)



## Documentation / Examples

- [x] Make sure the linting passes

***

Related to #23607

We should use `NEXT_PUBLIC_`  as [stated here](https://nextjs.org/docs/basic-features/environment-variables) to expose environment variables to the browser.

The code was already using the correct variables name.
azukaru/next.js,flybayer/next.js,zeit/next.js,JeromeFitz/next.js,JeromeFitz/next.js,JeromeFitz/next.js,azukaru/next.js,zeit/next.js,flybayer/next.js,azukaru/next.js,azukaru/next.js,zeit/next.js,flybayer/next.js,flybayer/next.js,JeromeFitz/next.js