docs: Fix path to MTD command line partition parser

cmdlinepart.c has been moved to drivers/mtd/parsers/.

Fixes: a3f12a35 ("mtd: parsers: Move CMDLINE parser")
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>