Update GPU admonition to match jstac's text

[mmcky]

Summary

Updates the GPU admonition to match jstac's improved text from lecture-python.myst PR #750.

Changes

Updated lectures/_admonition/gpu.md to use the numbered list format that's clearer for users:

This lecture was built using a machine with access to a GPU.

Google Colab has a free tier with GPUs that you can access as follows:

1. Click on the "play" icon top right
2. Select Colab
3. Set the runtime environment to include a GPU

Benefits

• Consistent with lecture-python.myst
• Clearer step-by-step instructions
• Links to Google Colab

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