Specified output fields separator

[zoltanf]

As per documentation the -p parameter takes input with field separated by commas, but in using it on my environment I have with trial and error figured that actually the field names should be space separated.

e.g.:

engage -p '$email,$first_name'
(does not work)

engage -p '$email $first_name'
(works fine)

Would be nice to fix in the doc so other people who run into this don't waste time.