Update jsonapi doc for 1.0 diner examples to use symbols as keys

[jessereynolds]

Otherwise you get:

irb(main):005:0> Flapjack::Diner.create_entities([{'id' => 'spongebob', 'name' => 'spongebob.example'}])
ArgumentError: 'id' is required.
    from /Users/jesse/src/flpjck/flapjack-diner/lib/flapjack-diner/argument_validator.rb:22:in `validate'
    from /Users/jesse/src/flpjck/flapjack-diner/lib/flapjack-diner/resources/entities.rb:15:in `block in create_entities'