Update README.md to correct case sensitive name (#1291)

The export for the @greenwood/plugin-postcss was incorrectly capitalized causing the import to fail if copied directly from the documentation.

packages/plugin-postcss/README.md

@@ -80,17 +80,17 @@ _Eventually once [PostCSS adds support for ESM configuration files](https://gith
 
 If you would like to _extend_ the default configuration with your own custom _postcss.config.js_, you can enable the `extendConfig` option of this plugin
 ```js
-import { greenwoodPluginPostcss } from '@greenwood/plugin-postcss';
+import { greenwoodPluginPostCss } from '@greenwood/plugin-postcss';
 
 export default {
   // ...
 
   plugins: [
-    greenwoodPluginPostcss({
+    greenwoodPluginPostCss({
       extendConfig: true
     })
   ]
 };
 ```
 
-This will then process your CSS with PostCSS using the configured plugins / settings you provide, merged _after_ the default Greenwood configuration listed above.
+This will then process your CSS with PostCSS using the configured plugins / settings you provide, merged _after_ the default Greenwood configuration listed above.