When making a custom-theme, I accidentally hard-coded the edit-URL to
point to the index-file in the mesa3d.org repo instead of pointing to
the correct file in our docs. This fixes that, so the "Edit this
page"-links in the footer works the same way as the old "Edit on
GitLab"-links did.
Fixes: 7da0482636 ("docs: add custom html theme")
Reviewed-by: David Heidelberg <david.heidelberg@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22791>