[css-align] Fix reference to unexisting justify-items: auto. (w3c#1650)

Fixes w3c#1649.

Author: emilio

css-align/Overview.bs

@@ -1489,7 +1489,7 @@ Inline/Main-Axis Alignment: the 'justify-items' property</h3>
 	Inherited: no
 	Percentages: n/a
 	Media: visual
-	Computed value: specified value, except for ''justify-items/auto'' (see prose)
+	Computed value: specified value, except for ''justify-items/legacy'' (see prose)
 	Animatable: no
 	</pre>