tracing/documentation: Fix a typo in ftrace.txt

'ftrace' is no longer the name of the function tracer, to activate
the function trace 'echo function > current_tracer' is to be used instead
of 'echo ftrace > current_tracer'. Update the documentation to reflect
the current implementation.

Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
LKML-Reference: <4B5D0BA8.20106@cn.fujitsu.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>