nixos: Fix build of manual

Broken by 17389e25

.

The description attributes of mkOption are parsed by XSLT, so we can
create a DocBook manual out of it.

Unfortunately, the passwordHash option had a description which includes
a <password> placeholder which is recognized by DocBook XSL as a valid
start tag. So as there is obviously no </password>, the build of the
manual bailed out with a parsing error.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Reported-by: devhell <"^"@regexmail.net>