Fix references to NAN and INFINITY in documentation

[themichaelyang]

Fixes typos in the documentation for Float::INFINITY and Float::NAN to make the example code work.

This gem redefines Float::NAN as JSON::NaN and Float::INFINITY as JSON::Infinity, so the docs use the wrong casing.

I added both the Float:: and JSON:: variations to make it more clear.

Tested in irb on Ruby 3.4.0:

irb(main):001> JSON.generate([Float::NAN, Float::INFINITY, JSON::NaN, JSON::Infinity, JSON::MinusInfinity], allow_nan: true)
=> "[NaN,Infinity,NaN,Infinity,-Infinity]"

irb(main):002> JSON.generate([Float::Infinity], allow_nan: true)
(irb):2:in '<main>': uninitialized constant Float::Infinity (NameError)

irb(main):003> JSON.generate([Float::NaN], allow_nan: true)
(irb):3:in '<main>': uninitialized constant Float::NaN (NameError)