doc: add missing docbook parameter for fence-array

Fixes 'make htmldocs' warning.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161023093044.324edfb6@xeon-e3