ffplay : Add 's' to the list of supported commands.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@22823 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

ffplay.c

@@ -3074,6 +3074,7 @@ static void show_help(void)
            "v                   cycle video channel\n"
            "t                   cycle subtitle channel\n"
            "w                   show audio waves\n"
+           "s                   activate frame-step mode\n"
            "left/right          seek backward/forward 10 seconds\n"
            "down/up             seek backward/forward 1 minute\n"
            "mouse click         seek to percentage in file corresponding to fraction of width\n"