docs: fix wrong usage of proftool

The usage of proftool in docs is incorrect. If proftool is used without
'-o' argument, it will show the usage like following

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

Change '>' to '-o' to fix it.

Signed-off-by: Puhan Zhou <puh4n.zhou@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>