[css-grid-3] Fix computed value w3c#9328

Author: tabatkins

css-grid-3/Overview.bs

@@ -1017,7 +1017,7 @@ Placement Precision: the 'masonry-slack' property</h3>
 	Percentages: relative to the [=grid-axis=] [=content box=] size of the [=masonry container=]
 	Inherited: no
 	Applies to: [=masonry containers=]
-	Computed value: a computed length
+	Computed value: a computed <<length-percentage>> value
 	Animation type: as length
 	</pre>