gh-96947: Fix comment on `pyruntimestate->pyinterpreters` struct for `next_id` (GH...

gh-96947: Fix comment on `pyruntimestate->pyinterpreters` struct for `next_id` (GH-96949)

`_next_interp_id` appeared on [this commit](https://github.com/python/cpython/commit/e377416c10eb0bf055b0728cdcdc4488fdfd3b5f#diff-7ac11e526f79b42d6ea9d3592cb99da46775640c69fa5510f4a6de87cced7141R68) renamed to `next_id` ([by this commit](https://github.com/python/cpython/commit/2ebc5ce42a8a9e047e790aefbf9a94811569b2b6#diff-bccfc01bd96b58c022dde77486b8a896cbb31d7581bd4a4156b32c3654afe468R59)).

Also, now, `next_id` gets initialized in` _PyInterpreterState_Enable()` https://github.com/python/cpython/blob/12c5f328d2479ac3432df5e266adc4e59adeabfe/Python/pystate.c#L241-L244 because `_PyInterpreterState_Init()` function doesn't exist at all.