Specify format as audio/opus instead of audio/webm

Previously, when using download feature, opus audio would be listed as
audio/webm. This may be confusing and so exact codec is specified.

src/invidious/frontend/watch_page.cr

@@ -74,6 +74,9 @@ module Invidious::Frontend::WatchPage
 
       video_assets.audio_streams.each do |option|
         mimetype = option["mimeType"].as_s.split(";")[0]
+        if mimetype == "audio/webm"
+          mimetype = "audio/opus"
+        end
 
         value = {"itag": option["itag"], "ext": mimetype.split("/")[1]}.to_json