docs: fix default user-agent example being outdated

is it fine that this is wrapped? :p

src/core/config/mod.rs

@@ -1854,7 +1854,8 @@ pub struct Config {
 
 	/// User agent that is used specifically when fetching url previews.
 	///
-	/// default: "continuwuity/<version> (bot; +https://continuwuity.org)"
+	/// default: "continuwuity/<version> (embedbot; facebookexternalhit/1.1; 
+	/// +https://continuwuity.org)"
 	pub url_preview_user_agent: Option<String>,
 
 	/// Determines whether audio and video files will be downloaded for URL