nixos: Fix build of manual

Manual build broken by 79f7f894

, which
is part of pull request #59179 (Fix Cassandra, improve config and
tests).

The issue was just a small error because of an unbalanced <literal/>
tag, so only a "/" was missing :-)

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @aanderse