Fix link_to_reply_diff.

Came from a typo `<` instead of `<<`.

@randx consider using https://github.com/scrooloose/syntastic and `gem install ruby-lint`: it put a huge yellow marker next to that line saying statement without effect. (does some false positives too, but I think it's worth it).

View changes: before:

![screenshot from 2014-09-19 14 06 51 gitlab reply diff before](https://cloud.githubusercontent.com/assets/1429315/4334966/5e524ca0-3ff6-11e4-843c-8692ac2291ed.png)

After:

![screenshot from 2014-09-19 14 06 32 gitlab reply diff](https://cloud.githubusercontent.com/assets/1429315/4334970/6a85a7d8-3ff6-11e4-8ef5-32d728800f79.png)


Prefer single-quoted strings when you don't need string interpolation or special symbols.