message: docs(style-props): add background shorthand
diff: @@ -231,7 +231,7 @@ import { Box } from "@chakra-ui/core"
  | Prop | CSS Property | Theme Key |
  | ------------------------------------- | ----------------------- | --------- |
  -| `background` | `background` | none |
  +| `bg`, `background` | `background` | none |
  | `bgImage`, `backgroundImage` | `background-image` | none |
  | `bgSize`, `backgroundSize` | `background-size` | none |
  | `bgPosition`,`backgroundPosition` | `background-position` | none |
  