[master] fix xsltproc --stringparam html.stylesheet usage
This is to set the location of the css stylesheet. Previously it pointed to a path in the source directory. By default, this was "./" so was okay as a location via HTTP. But when built using a different source directory, then the full source directory ended up as a reference on the webpage so then style's didn't work. I briefly discussed this on jabber yesterday but didn't get an okay. It is a minor fix, so committing per my discretion.