RFR: JDK-8157253: Fix compare script for html files

[Tim Bell]

Erik:

> While doing work with the generated docs image, I noticed that the
> compare script needs some tweaking for comparing docs correctly.
>
> Bug:https://bugs.openjdk.java.net/browse/JDK-8157253
> Webrev:http://cr.openjdk.java.net/~erikj/8157253/webrev.top.01/  <http://cr.openjdk.java.net/%7Eerikj/8157253/webrev.top.01/>

Looks good.

Tim