Update shCore.css

Removed a CSS rule which made line number and line content be aligned differently, vertically; the .container div had extra redundant space at the top. The issue was visible on Firefox only. Screenshot attached.

![screen_div](https://f.cloud.github.com/assets/1086929/2063850/6601fa04-8cbe-11e3-8b0e-ef53075888f0.png)