[fix] doc of locales.get_engine_locale() / zh-classical is missleading

Wikipedia's zh-classical is not zh_Hant (see doc-string of engines.wikipedia).
Fixed the example in the doc-string of locales.get_engine_locale() to 'zh_TW'.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>