docs(migration): fix syntax for main.js step
docs(migration): fix syntax for main.js step

- also add a test property to the configuration
otherwise the build takes way longer than it should

kadirahq/react-storybook,storybooks/storybook,storybooks/storybook,storybooks/react-storybook,kadirahq/react-storybook,storybooks/storybook,storybooks/storybook,storybooks/storybook,storybooks/storybook,storybooks/storybook,storybooks/react-storybook,storybooks/react-storybook,storybooks/react-storybook