nixos: Fix build of manual
nixos: Fix build of manual

Commit 29d7d8f44df24d6876c527d442eb42aee7227d6b has introduced another
section with the ID "sec-release-19.09-incompatibilities", which
subsequently causes the build to fail.

I just merged both sections and the manual is now building again.

Signed-off-by: aszlig <ee1aa092358634f9c53f01b5a783726c9e21b35a@nix.build>

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