Fixed readme example for COPY FROM.

The example would call the ```done`` function when the FileStream had finished rather than the stream to postgres had finished. This caused a test of mine to fail where after end checking the database had the correct row count, and the fix was to put the ```on('end'``` callback on the returned (piped) stream.

My code is here, https://github.com/grahamjenson/ger/blob/master/lib/psql_esm.coffee#L182

Cheers, Great project!