examples : fix nthread parsing in whisper.wasm (#2938)

examples : fix nthread parsing in whisper.wasm (#2938)

This commit fixes the nthread parsing in the whisper.wasm example when
using the `Threads` slider to change the number of threads to be used.

Currently this results in the following error:
```console
main.js:5597 Uncaught TypeError: Cannot convert "5" to int
    at checkAssertions (main.js:5597:21)
    at Object.toWireType (main.js:5611:15)
    at Object.full_default (eval at new_ (main.js:5292:27), <anonymous>:10:26)
    at whisper.wasm/:649:42
```