Fix order of operation when calculating margin for forms in rows

The calculation of margin for rows inside of forms is calculated incorrectly due to SASS interpreting `margin: 0 -$form-spacing / 2` as an arithmetic operation.  As a result the resulting CSS is `margin: -0.5em` (depending on what $form-spacing is). This fix makes the order of operation explicit so that the resulting CSS is as intended `margin: 0 -0.5em`