chore(hcl2cdk): fix yarn command in Readme.md

[bugzpodder]

Related issue

Fixes #

Description

yarn install is for installing existing packages, yarn add is for adding a new package.

Checklist

• I have updated the PR title to match CDKTF's style guide
• I have run the linter on my code locally
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation if applicable
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works if applicable
• New and existing unit tests pass locally with my changes

[hashicorp-cla-app]

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[github-actions]

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