Fix page build failed

The Jekyll pages failed to build the because of a missing angle bracket in `app/_layout/default.html` introduced in #2.

See error receive via email:
```
The page build failed with the following error:

A file was included in `./app/_layouts/default.html` that is a symlink or does not exist in your `_includes` directory. For more information, see https://help.github.com/articles/page-build-failed-file-is-a-symlink.

If you have any questions please contact us at https://github.com/contact.
```