[css-images-4] Fix off-by-one error.

Author: fantasai

css-images-4/Overview.bs

@@ -538,7 +538,7 @@ Combining images: the ''cross-fade()'' notation {#cross-fade-function}
 	</pre>
 
 	The function represents an image generated by
-	combining two or more images.
+	combining one or more images.
 
 	The <<percentage>> represents how much of each image is retained
 	when it is blended with the other images.