Fix a duplicate of <mark> styling

The content of the file `typography/_dfn.scss` is exactly a duplicate of the content of the file `typography/_mark.scss`. This pull request restores the styling of `_dfn.scss` according to [normalize.scss](http://git.io/normalize).
Merge pull request #3 from byjml/patch-1

Fix the styling of <dfn> in _dfn.scss