docs: update redis-cache.md

[quul]

Netlify functions' default region(and only available region to free accounts) is now AWS us-east-2 ref: https://docs.netlify.com/functions/overview/#default-deployment-options

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[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (984ea7a) 76.56% compared to head (c1be9ed) 76.56%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #243 +/- ## ======================================= Coverage 76.56% 76.56% ======================================= Files 61 61 Lines 2210 2210 Branches 544 544 ======================================= Hits 1692 1692 Misses 343 343 Partials 175 175 ```

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[geekdada]

感谢！