Docs: Correct typo in rollup example (#2406)

Rollup is imported by name, so it should be called like `rollup()`.

Author: aaronhooper

docs/getting-started/7-using-plugins.md

@@ -37,7 +37,7 @@ const { rollup } = require('rollup');
 
 // Rollup's promise API works great in an `async` task
 exports.default = async function() {
-  const bundle = await rollup.rollup({
+  const bundle = await rollup({
     input: 'src/index.js'
   });