Small fix on COPY ON_ERROR document

Hi,

I found that the documentation of COPY ON_ERROR said
COPY stops operation at the first error when 
"ON_ERROR is not specified.", but it also stop when
ON_ERROR is specified to the default value, "stop".

I attached a very small patch to fix this just for
making the description more accurate.

Regards,
Yugo Nagata 

-- 
Yugo NAGATA <nagata@sraoss.co.jp>