[css-grid] Add missing animatable line

Author: fantasai

css-grid/Overview.bs

@@ -990,6 +990,7 @@ Track Sizing: the 'grid-template-rows' and 'grid-template-columns' properties</h
 	Percentages: refer to corresponding dimension of the content area
 	Media: visual
 	Computed value: As specified, with lengths made absolute
+	Animatable: as a <a href="http://www.w3.org/TR/css3-transitions/#animtype-simple-list">simple list</a> of <a href="http://www.w3.org/TR/css3-transitions/#animtype-lpcalc">length, percentage, or calc</a> components
 	</pre>
 
 	These properties specify,