ftrace: Fix spelling mistake "disabed" -> "disabled"

There is a spelling mistake in a comment, fix it.

Link: https://lkml.kernel.org/r/20210311094022.5978-1-colin.king@canonical.com

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>