Use stylesheet_url tag

It looks like the syntax for a stylesheet tag in liquid is incorrect. This should fix the main stylesheet by using the liquid tag [stylesheet_tag](http://docs.shopify.com/themes/liquid-basics/output#stylesheet-tag).

Alternatively, if you're more comfortable using vanilla HTML and CSS, you could use a regular stylesheet link and the liquid [asset_url](http://docs.shopify.com/themes/filters/asset-url).
```html
<link rel="stylesheet" href="{% 'main.css' | asset_url %}" />
```
 :smile: