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>

Documentation/admin-guide/kernel-parameters.txt

@@ -2791,7 +2791,7 @@
 			<name>,<region-number>[,<base>,<size>,<buswidth>,<altbuswidth>]
 
 	mtdparts=	[MTD]
-			See drivers/mtd/cmdlinepart.c.
+			See drivers/mtd/parsers/cmdlinepart.c
 
 	multitce=off	[PPC]  This parameter disables the use of the pSeries
 			firmware feature for updating multiple TCE entries