texi2html: Use only one <pre class="example"> for @example -> valid HTML now

texi2html: Use only one <pre class="example"> for @example -> valid HTML now

The @example environment should not create
   <pre class="example"><pre class="example">...</pre></pre>
which is invalid HTML, but rather only one <pre> tag. texi2html creates
the correct pre tag automatically from the env name, so remove it from
the settings.
Also adjust the css to get the correct margins/padding like before.

Now, the lilypond-big-file has only 3 HTML errors left instead of ~300.
(cherry picked from commit c55f7c4cd738e7123ae55649920fac39d0e9e9d6)