Removed an unnecessary css value

Using one less css value for .hero-unit's margin generates the same outcome.
1 value : 'Top/Right/Bottom/Left',
2 values: 'Top/Bottom' + 'Left/Right',
3 values: 'Top' + 'Left/Right' + 'Bottom',
4 values: 'Top' + 'Right' + 'Bottom' + 'Left';