Fix the JSON::Coder.new call-seq

[dduugg]

The JSON::Coder#initialize call-seq is inaccurate in two ways:

# :call-seq:
#   JSON.new(options = nil, &block)

JSON is a module and has no .new, so the name should be JSON::Coder.new. More substantively, the positional options = nil has not been accurate since 3.0.0 (28a2b0b), which changed the signature to initialize(object_class: nil, array_class: nil, on_load: nil, **options, &as_json). Passing options positionally now raises:

$ ruby -rjson -e 'JSON::Coder.new({ symbolize_names: true })'
-e:1:in 'JSON::Coder#initialize': wrong number of arguments (given 1, expected 0) (ArgumentError)

whereas it worked on 2.x, so the documented form is the one that no longer works. The prose immediately below ("Argument options, if given, contains a Hash of options") has the same problem, so this updates it to describe keyword arguments.

This survived the 3.0 docs pass in 8bdd517, and it is currently what https://docs.ruby-lang.org/en/master/JSON/Coder.html renders. It misled me while migrating Homebrew off json/add/exception, which is the migration CHANGES.md recommends JSON::Coder for, so it seems worth correcting now that more people will be reading this method.

Documentation only, no behaviour change.