Update pip install command for agent-framework

[sphenry]

Motivation and Context

Description

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull Request Overview

Updates the pip install command in the README documentation to use the standard --pre flag instead of the non-standard --prerelease=allow flag for installing prerelease versions of the agent-framework package.

• Replaces incorrect pip flag syntax with the standard prerelease installation option

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