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docs: replace `var` with `const` (#465)

Replace `var` with `const` in the code example in README.
`const` is better, and since `const` and `var` were mixed up, I unified them into `const`.
T
Tomoaki Abe committed
33b66742498e07bbaaaaf0815f8cf2899d86a583
Parent: 25f4821
Committed by GitHub <noreply@github.com> on 7/16/2021, 8:01:13 AM