message: update BaseCSS usage instructions
diff: @@ -38,7 +38,8 @@ This project uses [emotion] under the hood to generate static CSS from _some_ co
  To ensure proper styling, you'll need to include an instance of the `BaseCSS` component somewhere in your document (preferably inside the `<head>`):
  ```jsx
  -import {BaseCSS, Heading} from 'primer-react'
  +import BaseCSS from 'primer-react/css'
  +import {Heading} from 'primer-react'
  export default () => (
  <html>
  @@ -55,7 +56,7 @@ export default () => (
  If you can't use React to render your document shell server-side, you have two other options:
  1. Include `node_modules/primer-react/dist/css/build.css` by copying or other means.
  -1. `import {BaseCSS} from 'primer-react'` and output the `BaseCSS.css` CSS string directly.
  +1. `import {css} from 'primer-react/css'` and output the `css` string directly.
  #### Static CSS rendering
  