Replace Decimal litaral with Rational literal in past entries [ja]

In some past entries about Ruby 2.1.0, one of new literals was described as `Decimal` (小数).
However it may mean `Rational` (有理数). (ref [https://bugs.ruby-lang.org/issues/8430](https://bugs.ruby-lang.org/issues/8430))

Note:
In some entries in English (and all other languages), there are descriptions about `Decimal` literal, but it may be also `Rational`.
So we should fix  other language versions about it.