Corrected the mention string in the user stream.

When you mention somebody, and then check their profile, it will not correctly show their name. Instead, it will show your own name (though the profile URL would be correct). The problem was in the localization strings under key: `you_mentioned_user`. I went ahead and fix that. I skimmed through the languages to make sure it was appropriate, and aside from Korean I can pretty much say it is for all of them.

Here is a screenshot of the problem:

![screen shot 2013-11-26 at 23 20 36](https://f.cloud.github.com/assets/1847066/1626932/6a5e7902-56e9-11e3-95bc-36ab07f81b58.png)

This is me on @eviltrout profile on [Ember forums](http://discuss.emberjs.com).