docs: ftrace: Correct access mode

The documentation gives an example for opening trace marker with
write-only mode, but the flag WR_ONLY is not defined by glibc.

Use O_WRONLY to replace it.
Signed-off-by: NLeo Yan <leo.yan@linaro.org>
Acked-by: NSteven Rostedt (Google) <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/20221008083250.3160-1-leo.yan@linaro.orgSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/trace/ftrace.rst

@@ -564,7 +564,7 @@ of ftrace. Here is a list of some of the key files:
 
 	start::
 
-		trace_fd = open("trace_marker", WR_ONLY);
+		trace_fd = open("trace_marker", O_WRONLY);
 
 	Note: Writing into the trace_marker file can also initiate triggers
 	      that are written into /sys/kernel/tracing/events/ftrace/print/trigger