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

include/linux/fence-array.h

@@ -53,6 +53,7 @@ extern const struct fence_ops fence_array_ops;
 
 /**
  * fence_is_array - check if a fence is from the array subsclass
+ * @fence: fence to test
  *
  * Return true if it is a fence_array and false otherwise.
  */