fix doc error

[baurine]

If we use as the following code in the doc:

const esbuild = require("esbuild");
const postCssPlugin = require("esbuild-plugin-postcss2");

esbuild.build({
  ...
  plugins: [
    postCssPlugin.default()
  ]
  ...
});

It will report the following error:

$ node builder.js
/repo_path/node_modules/esbuild-plugin-postcss2/dist/index.js:37
  plugins = [],
  ^

TypeError: Cannot read properties of undefined (reading 'plugins')
    at Object.postCSSPlugin (/mnt/bao/codes/personal/try-esbuild/esbuild-demo/node_modules/esbuild-plugin-postcss2/dist/index.js:37:3)
    at Object.<anonymous> (/mnt/bao/codes/personal/try-esbuild/esbuild-demo/builder.js:41:34)
    at Module._compile (node:internal/modules/cjs/loader:1101:14)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1153:10)
    at Module.load (node:internal/modules/cjs/loader:981:32)
    at Function.Module._load (node:internal/modules/cjs/loader:822:12)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12)
    at node:internal/main/run_main_module:17:47
error Command failed with exit code 1.

[baurine]

hi @martonlederer , the commit 32d928a is trying to fix this issue, right? but it seems the CI test doesn't pass, can you have a look? thanks!

[baurine]

This PR can be closed without merging if PR #30 is merged.