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

[netlify]

✅ Deploy Preview for surgio-documentation ready!

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[codecov]

Codecov Report

All modified lines are covered by tests ✅

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

Additional details and impacted files

@@           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！