Add additional cases to object styles example

[samsch]

Here -> https://styled-components.com/docs/advanced#style-objects

Same stuff the example could clear up with some tweaks:

  backgroundColor: 'red',
  // or
  'background-color': 'red',

  // does stuff like this work?
  backgroundColor: props => props.background,
  ':hover': props => css`background-color: red;`,