docs: Correct `import` for latest `autoprefixer`

[chris-dura]

When following the docs for @stencil/postcss on a relatively barebones project (all I did was add @stencil/sass as well), I get a compiler error:

> stencil-1-8-7@0.0.1 build /Users/CDura/code/stencil-1-8-7
> stencil build --docs

[ ERROR ]  autoprefixer is not a function TypeError: autoprefixer is not a function at Object.<anonymous>
           (/Users/CDura/code/stencil-1-8-7/stencil.config.ts:31:23) at Module._compile
           (internal/modules/cjs/loader.js:955:30) at Object.require.extensions..ts
           (/Users/CDura/code/stencil-1-8-7/node_modules/@stencil/core/dist/sys/node/index.js:4817:16) at Module.load
           (internal/modules/cjs/loader.js:811:32) at Function.Module._load (internal/modules/cjs/loader.js:723:14) at
           Module.require (internal/modules/cjs/loader.js:848:19) at require (internal/modules/cjs/helpers.js:74:18) at
           requireConfigFile
           (/Users/CDura/code/stencil-1-8-7/node_modules/@stencil/core/dist/sys/node/index.js:4820:20) at
           loadConfigFile (/Users/CDura/code/stencil-1-8-7/node_modules/@stencil/core/dist/sys/node/index.js:4771:32)
           at NodeSystem.loadConfigFile
           (/Users/CDura/code/stencil-1-8-7/node_modules/@stencil/core/dist/sys/node/index.js:33404:16)

I'm guessing autoprefixer changed how they're exporting their members. Because removing the wildcard * fixed it for me.

Updating the docs so users don't bang their heads against the wall. :)

[chris-dura]

derp. #20 already exists 🤦‍♂