README viewer in package homepage should be improved

The code in ```js block is displayed ugly.

This is because the spaces in the <pre> element was replaced to &nbsp;, but this Unicode is in double width under style font-family:monospace.

Just not do that, the problem will be resolved. Because the <pre> element’s style is white-space: pre. Or set the font-family other values, which render &nbsp; to 1en width.