nixos: Fix build of the manual

The manual still had a reference to an option that was already renamed
ages ago and a7ed44cc

 made it 'visible'.

With the visible attribute set to false for extraKernelModules, the
option no longer appears in the manual and thus breaks the link from the
manual to the options.

This is easily fixed by referring to boot.initrd.kernelModules instead
of the obsolete option boot.initrd.extraKernelModules.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @rycee, @matthewbauer