[DOC] Use `Ruby::Box#require_relative` in box.md examples

Based on the example, it appears that `foo.rb` and `main.rb` are expected to be in the same directory.
Since Ruby 1.9, the current directory is not included in `$LOAD_PATH` by default.
As a result, running `box.require('foo')` as shown in the sample code raises a `LoadError`:

```console
main.rb:2:in `Ruby::Box#require': cannot load such file -- foo (LoadError)
        from main.rb:2:in `<main>'
```

To avoid this, it seems simplest to show either `box.require('./foo')` or `box.require_relative('foo')`.
In this PR, `box.require('foo')` is replaced with `box.require_relative('foo')` to make the intention of
using a relative path explicit.

This should reduce the chance that users trying Ruby Box will run into an unexpected error.

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