fix: error option in async component migration page

[lovetingyuan]

Description of Problem

Wrong option field in async component migration page: https://v3.vuejs.org/guide/migration/async-components.html#_3-x-syntax.

Proposed Solution

change the wrong option name to correct ones.

Additional Information

[skirtles-code]

Looks good.

I'm unclear why that example is needed. It doesn't really add anything that isn't included in the previous example. With a small tweak to the wording of the sentence about the loader option, I think we could remove the example altogether.