docs: fix typos in Python examples

[sy0u1ti]

What does this PR do?

Corrects three typos in Python examples: a Haystack prompt, a LlamaIndex pipeline variable, and a Vertex AI sample prompt.

Why?

These examples are copied by users, so the prompt text and identifiers should be clear and correctly spelled.

How was it tested?

• Unit tests added / updated (N/A - example text and identifier corrections only)
• Integration tests pass (N/A - no gateway behavior changed)
• ruff check passes for all three changed files
• All three changed files compile with python -m py_compile
• codespell passes for all three changed files
• Public types, env vars, or SDK behavior changes are documented (N/A - none changed)

Checklist

• Branch is off main
• Commit messages follow Conventional Commits
• No TODOs or commented-out code left in
• No real API keys or secrets in the diff
• Prose changes do not introduce new project terminology

Notes for reviewers

No runtime behavior or public API changes.