FAQ: Fix dead link to old SPDX spec.

[andreashaerter]

As the title states: Fixed a dead link to SPDX documents.

Question: There are also several dead links to this resource in translations files:

~/dev/reuse-website$ grep "spdx-specification-21-web-version" -r -l
site/po/cs.po
site/po/es.po
site/po/fr.po
site/po/it.po
site/po/reuse-website.pot
site/po/ru.po

especially referencing

SPDX Specification, Appendix IV, https://spdx.org/spdx-specification-21-web-version#h.jxpfx0ykyb60

which should be replaced by

SPDX Specification, Annex D, https://spdx.github.io/spdx-spec/v2.3/SPDX-license-expressions/

Is it OK to edit these .po files directly? (I've never used GNU gettext myself, so this might be a dumb question.) If yes, I could add an addition commit to the PR. :)