Fix math rendering in PPO.md

[golden-panther]

AI Engineering Academy Pull Request

Description

Math was not rendering properly in PPO page.

Type of Change

• New learning content
• Content correction or improvement
• Code example or tutorial
• Infrastructure/deployment change
• Documentation update
• Other (please describe)

Educational Value

The content is more readable now.

Technical Details

Just markdown syntax change.

Testing Performed

I just viewed the markdown preview in VS code.

• Content review for accuracy
• Code examples have been tested
• Documentation builds correctly
• Links and references checked

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