Update docs to show after_build setting is off by default

Thanks for the gem, have a small doc change suggestion:

I believe that the current docs are incorrect in saying that the after build hook is on by default. I don't see it in the code and I think this issue https://github.com/karlfreeman/middleman-sync/issues/6 states that it is off by default.

This would have reduced some confusion I had when getting started.