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>