nixos tests.misc: unblock a man-page test

I'm not sure what's wrong, but the pages look OK. Discussion:
https://github.com/NixOS/nixpkgs/pull/27061#issuecomment-314330032

Changed files
+1 -1
nixos
tests
+1 -1
nixos/tests/misc.nix
···
};
subtest "nixos-rebuild", sub {
-
$machine->succeed("nixos-rebuild --help | grep SYNOPSIS");
+
$machine->succeed("nixos-rebuild --help | grep 'NixOS module' ");
};
# Sanity check for uid/gid assignment.