[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.

Author: koic

doc/language/box.md

@@ -117,7 +117,7 @@ Foo.foo.blank? #=> false
 
 # in main.rb
 box = Ruby::Box.new
-box.require('foo')
+box.require_relative('foo')
 
 box::Foo.foo_is_blank? #=> false   (#blank? called in box)
 
@@ -151,7 +151,7 @@ end
 
 # main.rb
 box = Ruby::Box.new
-box.require('foo')
+box.require_relative('foo')
 
 box::String.foo  # NoMethodError
 ```
@@ -174,7 +174,7 @@ Array.const_get(:V)              #=> "FOO"
 
 # main.rb
 box = Ruby::Box.new
-box.require('foo')
+box.require_relative('foo')
 
 Array.instance_variable_get(:@v) #=> nil
 Array.class_variable_get(:@@v)   # NameError
@@ -197,7 +197,7 @@ p $foo      #=> nil
 p $VERBOSE  #=> false
 
 box = Ruby::Box.new
-box.require('foo')  # "This appears: 'foo'"
+box.require_relative('foo')  # "This appears: 'foo'"
 
 p $foo      #=> nil
 p $VERBOSE  #=> false
@@ -216,7 +216,7 @@ Object::FOO #=> 100
 
 # main.rb
 box = Ruby::Box.new
-box.require('foo')
+box.require_relative('foo')
 
 box::FOO      #=> 100
 
@@ -241,7 +241,7 @@ yay     #=> "foo"
 
 # main.rb
 box = Ruby::Box.new
-box.require('foo')
+box.require_relative('foo')
 
 box::Foo.say  #=> "foo"