ftrace/documentation: fix typo in function grapher name

The function graph tracer is called just "function_graph" (no trailing
"_tracer" needed).

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
LKML-Reference: <1244623722-6325-1-git-send-email-vapier@gentoo.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>