Add missing ffplay subtitle cycling keybinding.

patch by Björn Axelsson, gecko acc.umu se


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@17078 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

doc/ffplay-doc.texi

@@ -107,6 +107,9 @@ Cycle audio channel.
 @item v
 Cycle video channel.
 
+@item t
+Cycle subtitle channel.
+
 @item w
 Show audio waves.