doc/openocd.texi: fix cross referencing

doc/openocd.texi: fix cross referencing

Commit b04c7c2ca587feebd6506716cd84297212350608 adds the cross
reference target "adapter_usb_location", but references it with
the command @xref inside a text, where command @ref should be
used. This triggers a compile warning:
	doc/openocd.texi:2517: warning: `.' or `,' must follow @xref, not c

Replace @xref with @ref and adapt both the text and the command
syntax.

Change-Id: I1520ffecb7d1d375f924492b1884d5b47f399007
Fixes: b04c7c2ca587 ("ftdi: use "adapter usb location" instead of
ftdi_location command")
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5045
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>