docs: replace deprecated llama 3 model with sonar in ChatPerplexity example (#31716)
**Description:** Updates ChatPerplexity documentation to replace
deprecated llama 3 model reference with the current sonar model in the
API key example code block.
**Issue:** N/A (maintenance update for deprecated model)
**Dependencies:** No new dependencies required
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Co-authored-by: Mason Daugherty <github@mdrxy.com>