python3: fix missing comma in get_module_deps3.py

Wes Tarro <wes.tarro@azuresummit.com> noticed a missing comma in a
preplace() call, add it.

That said, calling replace() with one argument results in a TypeError,
so this is obviously dead code.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>