docs(README): fix the gulp example

[wahidrizka]

The gulp example in the README could not run:

• It requires gulp-postcssrc, which is not on npm (the registry answers 404; Default option #18 calls it a proof of concept that "will stay unpublished"), so it fails with Cannot find module 'gulp-postcssrc'.
• The }) after the css function is left over from 0f26c60, when task('css', () => { ... }) became const css = () => { ... }, so Node stops with SyntaxError: Unexpected token ')'.
• css does not return the stream, so gulp reports The following tasks did not complete: default, css and Did you forget to signal async completion?, and watch never starts.

It now uses gulp-postcss, which loads postcss.config.js through postcss-load-config when it gets no plugins, and returns the stream.

I ran the fixed example with gulp 5.0.1, gulp-postcss 10.0.0 and the README's postcss.config.js, on a file with @import and nesting. With NODE_ENV=production it writes .imported{color:blue}.card{color:red}.card .title{margin:0}, with NODE_ENV=development the same CSS without cssnano, then watch starts and rebuilds after a change. The example from main fails with the SyntaxError above, and the fixed one fails as described without return or with gulp-postcssrc.