doc: issue tracking: add more vertical whitespace

  This change makes the first part of the file easier to read quickly.
It makes it also easier to spot the URLs which might be one of the main
interests of many readers.
Signed-off-by: Alexander Strasser <eclipse7@gmx.net>

doc/issue_tracker.txt

@@ -5,12 +5,15 @@ NOTE: This is a draft.
 
 Overview:
 ---------
+
 FFmpeg uses Trac for tracking issues, new issues and changes to
 existing issues can be done through a web interface.
+
 It is possible to subscribe to individual issues by adding yourself to the
 Cc list or to subscribe to the ffmpeg-trac mailing list which receives
 a mail for every change to every issue.
 (the above does all work already after light testing)
+
 The subscription URL for the ffmpeg-trac list is:
 http(s)://ffmpeg.org/mailman/listinfo/ffmpeg-trac
 The URL of the webinterface of the tracker is: