Typo in test_magic_number() comment

# Bug report

### Bug description:

#154881 introduced a typo:

```
# discussed in on Discourse. If a change is required, community
```

should be "on Discourse" not "in on Discourse"

### CPython versions tested on:

CPython main branch

### Operating systems tested on:

_No response_


### Linked PRs
* gh-154966