texi2html: Properly fix internal links in the big snippets page

texi2html: Properly fix internal links in the big snippets page

Don't rewrite html links to lilypond-snippets* in the snippets page.
The problem was that we used the regexp lilypond(|...), which was also
matched by lilypond-snippets. The solution (thanks to John) is to add a
negative lookahead to exclude -snippets...