Change yarn start to yarn dev and localhost:8000 to localhost:3000 in README file

[HiroyukiNaito]

Change yarn start to yarn dev and localhost:8000 to localhost:3000 in README file [Fixes #12039]

• Change command $ yarn start to $ yarn dev
• Change port localhost:8000 to localhost:3000`

Description

• Change description for running Next.js locallly

Related Issue

#12039

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[corwintines]

Thanks @HiroyukiNaito!

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