docs(codebuild): updated Linux build image return type in doc

[khushail]

Issue # (if applicable)

Closes #31717

Reason for this change

It should be in sync with the description as mentioned in here https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_codebuild.LinuxArmBuildImage.html

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[aws-cdk-automation]

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[khushail]

Exemption Request

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.

[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: bf962fb
• Result: SUCCEEDED
• Build Logs (available for 30 days)

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