Fix クラスメソッド -> クラスのメソッド in async-await/article.md

[HIRANO-Satoshi]

クラスメソッド -> クラスのメソッド

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[KenjiI]

ありがとうございます！ CLA への署名お願いできますでしょうか？ それが終わり次第マージさせていただきます。