Fixed typo

Fixed typo in CSS file 

Github seems to be flagging the entire file as changed even though I used their web interface to do it but suffice to say the only line edited was line 7 which went from

```font-family: 'Helvetica Neue', Helvetica, Arial, Sans-seris;```

to 

```font-family: 'Helvetica Neue', Helvetica, Arial, Sans-serif;```