docs: fix wrong proftool usage

Guide shows incorrect usage of proftool, which is confusing. If proftool
is used w/o '-o' argument it complains like following

$ ./sandbox/tools/proftool -m sandbox/System.map -t trace dump-ftrace > trace.dat
Must provide trace data, System.map file and output file
Usage: proftool [-cmtv] <cmd> <profdata>

s/>/-o/ fixes it and proftool outputs decoded data to trace.dat

Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/trace.rst b/doc/develop/trace.rst
index 8425d84..9bbe134 100644
--- a/doc/develop/trace.rst
+++ b/doc/develop/trace.rst
@@ -100,7 +100,7 @@
 
 .. code-block:: console
 
-    $ ./sandbox/tools/proftool -m sandbox/System.map -t trace dump-ftrace >trace.dat
+    $ ./sandbox/tools/proftool -m sandbox/System.map -t trace dump-ftrace -o trace.dat
 
 Finally run kernelshark to display it (note it only works with `.dat` files!):