Modify README.md.

The requirement that force just one content element is gone.

README.md

@@ -41,7 +41,6 @@ Requirements
 
 To make this plugin *perfect*, some requirements were not avoidable. But they're all very trivial and there's nothing to worry about.
 
-* there must be the *one* content element(like div) for the container.
 * the container must have a 'position' css style.
 * the scrollbar's position must be 'absolute'.
 * the scrollbar-x must have a 'bottom' css style, and the scrollbar-y must have a 'right' css style.