Minor documentation correction

Addresses #1755 .

So, the documentation regarding how to add fields to a project was apparently never right.  'field_name' should have been 'name' and 'units' should have been 'unit'.  I'm going to assume this isn't a breaking change to the API, considering that the API itself didn't change (and has never changed - it's been this way since 1/20/14 according to git).  The documentation now merely represents the universe we currently inhabit, and not the bizarro one where 'units' and 'field_name' are valid parameters used in creating a field.