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.