[BUG] Installation doc runs unrelated code with npx

[hukka]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Following the instructions at https://openapi-generator.tech/docs/installation, running npx openapi-generator generate -i petstore.yaml -g ruby -o /tmp/test/ will actually run code from
https://www.npmjs.com/package/openapi-generator, not https://www.npmjs.com/package/@openapitools/openapi-generator-cli.

This matches what npm would install with npm install --dry-run openapi-generator.

Suggest a fix

Remove npx from the instructions, or change it to use --package.

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