doc: Correct position of gdb '--args' parameter

The '--args' parameter to gdb comes before the binary that the debugger
will be attached to rather than after the binary and before the
arguments. Fix that in the docs.

Signed-off-by: Andrew Scull <ascull@google.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>