Doc: cull-idle-time parameter is defined in minutes

### Bug Description

There seems to be a minor bug in the documentation at https://charmhub.io/jupyter-controller/configurations#cull-idle-time

It specifies this parameter is taken in seconds. But in the actual config it seems to be in minutes.

Same can be confirmed here:
https://github.com/canonical/notebook-operators/blob/9e74713c6d9cd8d5189318784900934655cda49e/charms/jupyter-controller/config.yaml#L6C2-L9C134

### To Reproduce

This is a documentation bug.

### Environment

NA

### Relevant Log Output

```shell
NA
```

### Additional Context

_No response_

Compare the Charmhub cull-idle-time documentation with charms/jupyter-controller/config.yaml at the linked lines. Confirm the configuration unit is minutes, then update the documentation to match and verify that the parameter description no longer says seconds.

Written by the indexing model from the issue text.