Readme.md delete key does not exist

This is a small one, but, in the `README.md`:
```
export default {
  TODO_ITEM: {
    MOVE_LEFT: 'left',
    MOVE_RIGHT: 'right',
    MOVE_UP: ['up', 'w'],
    DELETE: {
      osx: ['command+backspace', 'k'],
      windows: 'delete',
      linux: 'delete',
    },
  },
}
```
the `delete` key is actually `del`.
