[fix] engines description - currency_convert.py

Currency engine has DuckDuckGo metadata

In the engine selector of the preferences window, the currency search engine has
the same metadata and wikidata url as duckduckgo, I'd assume there should be a
difference of some sort there clarifying what source the currency uses or, if
it's a duckduckgo service, at least clarifying that it's a currency service by
duck duck go.

Closes: https://github.com/searxng/searxng/issues/787


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