[IPBBridge] fix: bug in feed detection logic
[IPBBridge] fix: bug in feed detection logic

The previous author forgot to also append ".xml"
when actually fetching the detected feed.

RSS-Bridge/rss-bridge,ORelio/rss-bridge,kranack/rss-bridge,ORelio/rss-bridge,kranack/rss-bridge,Riduidel/rss-bridge,kranack/rss-bridge,ORelio/rss-bridge,Riduidel/rss-bridge,kranack/rss-bridge,Riduidel/rss-bridge,RSS-Bridge/rss-bridge,RSS-Bridge/rss-bridge