Fix typo in OpenAI Codex Introduction

[mikoflip]

Why:

This change fixes a minor spelling typo in the Introduction of the OpenAI Codex docs to improve readability and avoid confusion.

Closes: N/A (no issue filed; minor typo fix)

What's being changed (if available, include any code snippets, screenshots, or gifs):

Fix a small spelling typo in the Introduction of content/copilot/concepts/agents/openai-codex.md:

• subect → subject

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Table of review links

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Source	Review	Production	What Changed
copilot/concepts/agents/openai-codex.md	fpt ghec	fpt ghec

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[Copilot]

Pull request overview

This pull request fixes a spelling typo in the OpenAI Codex documentation, correcting "subect" to "subject" in the Introduction section.

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