[css-transitions-2] Change one occurrence of animatable to transitionable.

Author: dbaron

css-transitions-2/Overview.bs

@@ -77,7 +77,7 @@ and backport the following paragraph.
 
 Although the 'transition-property' may specify shorthand properties and the
 ''all'' keyword, individual transitions are generated for each longhand
-sub-property that is animatable. The <dfn>expanded transition property
+sub-property that is transitionable. The <dfn>expanded transition property
 name</dfn> of a transition is the name of the longhand sub-property for which
 the transition was generated (e.g. 'border-left-width').