Link to index in header leads to 404

The link to the index in the [header](https://github.com/MetaCoq/metacoq.github.io/blob/d7bb97ff6bf657c9cddbb44ee60bfd9c143c1063/html/resources/header.html#L22) leads to the non-existent [indexpage.html](https://metacoq.github.io/html/indexpage.html) instead of [index.html](https://metacoq.github.io/html/index.html).