Update changing-the-ai-model-for-copilot-code-completion.md

[dacrome]

Fix the intro to code completion instead of chat. I guess it was copied from the 'Change the chat model' page.

Why:

Closes: #36480

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

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copilot/using-github-copilot/ai-models/changing-the-ai-model-for-copilot-code-completion.md	fpt ghec	fpt ghec

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[hubwriter]

@dacrome - Many thanks for spotting and correcting this. 🎖️

I'll get this merged now.

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