Small comments on paper's links

[alecontri]

• The link to www.dlr-amr.github.io/t8code in the Summary leads to a 404 error page;
• The link to PADME-AM project leads to a conference abstract. Not necessary, but maybe it is better to link to the project page?

(Part of JOSS review https://github.com/openjournals/joss-reviews/issues/6887.)

[jmark]

Fixed.