Update SASS_REFERENCE.md

https://github.com/sass/sass/blob/stable/doc-src/SASS_REFERENCE.md

This example is producing invalid css. `font:` requires at least a font size and font family. http://www.w3schools.com/cssref/pr_font_font.asp
```
.funky {
  font: 2px/3px;
    font-family: fantasy;
    font-size: 30em;
    font-weight: bold; }
```
`.funky{ font: 2px/3px sans-serif;}` would be valid, but `.funky{ font: 2px/3px;}` is a style that your browser will chose to ignore.