append prompt in /debug/tracing/README file.

This is trivial issue. Fix typo Mini Howto file(README) for ftrace.

[ Impact: cleanup ]
Signed-off-by: NGeunSik Lim <geunsik.lim@samsung.com>
Acked-by: NSteven Rostedt <rostedt@goodmis.org>
Cc: williams <williams@redhat.com>
LKML-Reference: <1242289418.31161.45.camel@centos51>
Signed-off-by: NIngo Molnar <mingo@elte.hu>