kprobetrace, doc: Shell needs single quote to use $ character

Shell interprets $val as shell variable, thus we need quote if
we use the echo command.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
Cc: systemtap <systemtap@sources.redhat.com>
Cc: DLE <dle-develop@lists.sourceforge.net>
LKML-Reference: <20100119023505.31880.17367.stgit@localhost6.localdomain6>
Signed-off-by: Ingo Molnar <mingo@elte.hu>