chromium: Fix comment of upstream-info.nix

As of 6041cfe2

, the upstream-info.nix (back then it was called
sources.nix) is no longer in the source/ subdirectory, so we need to fix
that comment to say that the file is autogenerated from update.sh in the
*same* directory.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>