docs(contributing): correct codegen link

[turneand]

Issue

na

Description

The codegen link currently results in a 404 error, as the /codegen/sdk-codegen folder does not contain a README file.
This change simply moves the link to the codegen/README file which correctly explains how to run the scripts.

Testing

Verified in github new link points to a valid location that correctly explains codegen

Additional context

Add any other context about the PR here.

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[codecov-io]

Codecov Report

Merging #2060 (269ed55) into master (f2a47e8) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2060      +/-   ##
==========================================
+ Coverage   79.30%   79.34%   +0.04%     
==========================================
  Files         368      368              
  Lines       15132    15171      +39     
  Branches     3222     3242      +20     
==========================================
+ Hits        12001    12038      +37     
- Misses       3131     3133       +2     

Impacted Files	Coverage Δ
packages/util-waiter/src/utils/validate.ts	81.81% <0.00%> (-18.19%)	⬇️
lib/storage/src/data-chunk/readable-helper.ts	86.84% <0.00%> (-5.47%)	⬇️
packages/util-dynamodb/src/convertToAttr.ts	98.71% <0.00%> (-1.29%)	⬇️
packages/util-waiter/src/poller.ts	100.00% <0.00%> (ø)
packages/util-waiter/src/waiter.ts	100.00% <0.00%> (ø)
protocol_tests/aws-ec2/endpoints.ts	81.48% <0.00%> (ø)
protocol_tests/aws-json/endpoints.ts	81.48% <0.00%> (ø)
packages/middleware-ssec/src/index.ts	82.60% <0.00%> (ø)
protocol_tests/aws-query/endpoints.ts	81.48% <0.00%> (ø)
packages/util-dynamodb/src/marshall.ts	100.00% <0.00%> (ø)
... and 49 more

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[aws-sdk-js-automation]

AWS CodeBuild CI Report

• CodeBuild project: sdk-staging-test
• Commit ID: 269ed55
• Result: SUCCEEDED
• Build Logs (available for 30 days)

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[github-actions]

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