tutorials and screencasts pages: remove space before colon for legibility and remove broken link to username

There aren't public user pages to link to, so I removed the link altogether.

screenshots:
![image](https://f.cloud.github.com/assets/47234/1047247/e67e7f78-105d-11e3-81e6-156fbd7a444c.png)
![image](https://f.cloud.github.com/assets/47234/1047250/fdae0c18-105d-11e3-8545-5a51726d27d6.png)