doc: fix makeinfo warning

doc: fix makeinfo warning

Build returns a makeinfo warning:
	openocd.texi:5201: warning: `.' or `,' must follow @xref, not f

Add a dummy ',' after '@xref{..}' to silent the warning.

Fixes: 44cfdef0a40d ("server/gdb: Restructure commands")
Change-Id: Ic0bff8fc9b54942ebb72762816686ea7c7881345
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8392
Tested-by: jenkins
Reviewed-by: zapb <dev@zapb.de>