Moved minus sign inside parens, so calc would work.

Top and bottom margins on `block-grid-*` where being overridden by side margins because the minus sign was outside of the parenthesis, causing the calculation to eat up to `0px` top and bottom value:

Base code:
`margin: 0 -($spacing/2);`

Results in:
`margin: -0.625em;`

New code:
`margin: 0 (-$spacing/2);`

Results in:
`margin: 0 -0.625em;`