Fix ParallelHelper sample

[stevemonaco]

What changes to the docs does this PR provide?

Add readonly modifier to private field for the ArrayInitializer sample. Will not compile without it.

PR Checklist

Please check if your PR fulfills the following requirements:

• [✓ ] Correctly picked the right branch to base the change off (dev for new features, master for typos/improvements)
• [N/A ] For new pages, used the provided template
• [N/A ] For new features, added an entry in the Table of Contents
• [N/A ] Ran against a spell and grammar checker
• [ ✓] Contains NO breaking changes

[opbld31]

Docs Build status updates of commit 359259d:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
docs/high-performance/ParallelHelper.md	:white_check_mark:Succeeded	View

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