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>

Documentation/trace/ftrace.txt

@@ -179,7 +179,7 @@ Here is the list of current tracers that may be configured.
 
 	Function call tracer to trace all kernel functions.
 
-  "function_graph_tracer"
+  "function_graph"
 
 	Similar to the function tracer except that the
 	function tracer probes the functions on their entry