[css-transforms-2] Remove reference to nonexistent definition.

This fixes this bikeshed warning:
  LINE ~417: No 'dfn' refs found for 'flattening element'.
  [=flattening element|flattening=]
which removes the remaining link to a definition that no longer exists
(and was somewhat suspicious when it did).

Author: dbaron

css-transforms-2/Overview.bs

@@ -414,7 +414,7 @@ This example shows show elements in a 3D rendering context can intersect. The co
 	</div>
 	</pre>
 
-This example shows how nested 3D transforms are rendered. The blue div is transformed as in the previous example, with its rendering influenced by the perspective on its parent element. The lime element also has a 3D transform, which is a rotation about the X axis (anchored at the top, by virtue of the transform-origin). However, the lime element is being rendered into the plane of its parent because it is not a member of the same 3D rendering context; the parent is [=flattening element|flattening=]. Thus the lime element only appears shorter; it does not "pop out" of the blue element.
+This example shows how nested 3D transforms are rendered. The blue div is transformed as in the previous example, with its rendering influenced by the perspective on its parent element. The lime element also has a 3D transform, which is a rotation about the X axis (anchored at the top, by virtue of the transform-origin). However, the lime element is being rendered into the plane of its parent because it is not a member of the same 3D rendering context. Thus the lime element only appears shorter; it does not "pop out" of the blue element.
 
 	<div class="figure">
 		<img src="examples/3d-rendering-context-flat.png" width="240" height="200" alt="Nested 3D transforms, with flattening">