Fix the docs to match actual behavior of EINHORN_FD_N vars

They actually count from 0, not 1. I hadn't originally intended for
this to be the behavior, but now that the code is written, it seems
preferable to attempting to do a bunch of conversions between
0-indexing and 1-indexing, so I think I'd prefer to keep it.
Merge pull request #20 from ebroder/fix-envvar-docs

Fix the docs to match actual behavior of EINHORN_FD_N vars