Fixed 'grunt run' on Windows

There was a typo and I had to add quotes around the command so Windows
could find it.

Before fixing, I got this error:

```bash
$ grunt run
Building 0.1.1-beta

Running "run" task

Running "exec:win" (exec) task
>> 'build' is not recognized as an internal or external command,
>> operable program or batch file.
>> Exited with code: 1.
Warning: Task "exec:win" failed. Use --force to continue.

Aborted due to warnings.
```