Link to ModifyMeta.hs in documentation

[twsh]

In the documentation here there is a broken link to ModifyMeta.hs: https://github.com/lierdakil/pandoc-crossref/blob/master/lib/Text/Pandoc/CrossRef/Util/ModifyMeta.hs. The correct link is this, I think: https://github.com/lierdakil/pandoc-crossref/blob/master/lib-internal/Text/Pandoc/CrossRef/Util/ModifyMeta.hs