[DOC] Tweak an example in language/box.md

Although the example code comments indicate that it returns `false`,
a non-matching result for `=~` is actually `nil`.

```ruby
Foo.foo.blank? #=> false
"foo".blank?   #=> false
```

https://github.com/ruby/ruby/blob/v4.0.0-preview3/doc/language/box.md?plain=1#L115-L122

This PR replaces `=~` with `match?` so that it returns the expected `false`.
Since this makes the result a boolean, it also aligns with the expected behavior of
a predicate method name like `blank?`.

Author: koic

doc/language/box.md

@@ -100,7 +100,7 @@ The changed definitions are visible only in the box. In other boxes, builtin cla
 class String
   BLANK_PATTERN = /\A\s*\z/
   def blank?
-    self =~ BLANK_PATTERN
+    self.match?(BLANK_PATTERN)
   end
 end