doc: correctly terminate @xref

@xref should be terminated by either a ',' or '.'.

Change-Id: I141335cd4e149992fc971c6f2c04f1dbe390c8f4
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1637


Tested-by: jenkins
Reviewed-by: Xiaofan <xiaofanc@gmail.com>