libata: fix docs, RE port and device of libata.force ID separated by point

According to libata-core correctly around line 6572:

	/* parse id */
	p = strchr(id, '.');
	...

the optional device is separated from the port in the libata.force ID
by a point or dot instead of by a colon.

Fix documentation to reflect this.
Signed-off-by: Roman Fietze <roman.fietze@telemotive.de>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

Documentation/kernel-parameters.txt

@@ -1194,7 +1194,7 @@ and is between 256 and 4096 characters. It is defined in the file
 
 	libata.force=	[LIBATA] Force configurations.  The format is comma
 			separated list of "[ID:]VAL" where ID is
-			PORT[:DEVICE].  PORT and DEVICE are decimal numbers
+			PORT[.DEVICE].  PORT and DEVICE are decimal numbers
 			matching port, link or device.  Basically, it matches
 			the ATA ID string printed on console by libata.  If
 			the whole ID part is omitted, the last PORT and DEVICE