Fix formatting of path in README.md (#19407)

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## Summary

quotation mark is not closed in README doc of the postcss preprocesor

## Test plan

not needed

packages/@tailwindcss-postcss/README.md

@@ -49,7 +49,7 @@ import tailwindcss from "@tailwindcss/postcss"
 export default {
  plugins: [
   tailwindcss({
-    base: path.resolve(__dirname, "./path)
+    base: path.resolve(__dirname, "./path")
   })
  ]
 }