Is incorrect to have the HTTPS version of the sitemaps.org URL in the xmlns sitemap schema?

The value of the xmlns attribute is to uniquely identify a namespace (the "namespace URI"). It does not relate to whether your site is serving content over HTTP or HTTPS - for this it makes no difference.

The sitemaps.org site defines the protocol using http://www.sitemaps.org/schemas/sitemap/0.9 as the recognised namespace - so this should really be the string used.

(However, sitemaps.org does universally redirect everything to HTTPS, so maybe both would seem to be valid? But see @unor's comment below.)

You can validate your sitemap on https://www.google.com/webmasters/tools. It will give an error when you use https to refer to the schema.

I have also seen other sitemap validators that refuse to validate the https version of the namespace, so using http might be the only correct option, even when your site only uses https.