tests: import-validation docstring cites brave default_params.time_period, which never gained an options list

Follow-up to #5597 (fixes #5589). The module docstring of `tests/settings/test_import_settings_validation.py:6` cites the motivating example as: > a free-text `search.engine.web.brave.default_params.time_period` value from before the field gained an options list But that key has not gained an options list. In the current defaults: | key | `ui_element` | `options` | |---|---|---| | `search.engine.web.brave.default_params.time_period` | `text` | `null` | | `search.engine.web.serpapi.default_params.time_period` | `text` | `null` | | `search.engine.web.serper.default_params.time_period` | `select` | day / week / month / year / null | | `search.engine.web.tavily.default_params.time_period` | `select` | d / w / m / y | **Impact:** a maintainer tracing the regression to the cited key finds brave's `time_period` is still free-text, which undercuts the documented motivation and sends reproduction effort to the wrong setting. **Scope:** docstring only. The tests themselves correctly exercise `search.time_period` and the validation logic is unaffected. **Suggested fix:** point the example at a key that actually carries an options list — `search.engine.web.serper.default_params.time_period` or the tavily equivalent — or drop the engine qualifier so the example matches the changelog's generic `default_params.time_period` phrasing.