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]

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[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。