Update SASS_REFERENCE.md

This example is producing invalid css.

```
.funky {
  font: 2px/3px;
    font-family: fantasy;
    font-size: 30em;
    font-weight: bold; }
```
`font:` requires at least 1) a font-size and 2) a font-family: http://www.w3schools.com/cssref/pr_font_font.asp

`.funky{ font: 2px/3px fantasy;}` would be valid, but `.funky{ font: 2px/3px;}` is a style that your browser will chose to ignore. Best to not have invalid css in the docs. Any code examples should be copy/paste-able.

![screen shot 2014-08-01 at 5 30 16 pm](https://cloud.githubusercontent.com/assets/1933021/3786577/3818692e-19dc-11e4-878a-a11a026160ee.png)