Clarify uglyurls flag.

Mention more clearly that, for generating `/filename.html`, you need to
set the `uglyurls` flag to `true`.

From the initial documentation, I got the false impression that  
`--uglyurls=false` would "use `/filename.html` instead of `/filename/`".

But that is incorrect: the `=false` is the default value, and, **if true**, then the description applies.