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>