docs: fix typo in per-turn user simulator quality prompt

[anxkhn]

Please ensure you have read the contribution guide before creating a pull request.

Link to Issue or Description of Change

2. Or, if no issue exists, describe the change:

Problem:
The with-persona evaluator prompt template in
src/google/adk/evaluation/simulation/per_turn_user_simulator_quality_prompts.py
misspells "existing" as "exsisting" in its "Definition of Conversation History"
section. This text is part of the prompt sent to the model at evaluation time,
so it is model-facing rather than an internal comment. The sibling non-persona
template in the same file already uses the correct spelling in the identical
sentence, so the two templates are inconsistent.

Solution:
Correct the single word exsisting -> existing on that line. This makes the
two templates consistent and fixes the model-facing text. No code path changes.

Testing Plan

This is a small typo fix in a prompt string, so no behavioral tests are added.
The existing module test suite still passes and linting is clean:

• pre-commit run --files src/google/adk/evaluation/simulation/per_turn_user_simulator_quality_prompts.py -> all Passed/Skipped.
• pytest tests/unittests/evaluation/simulation/test_per_turn_user_simulation_quality_prompts.py -q -> 6 passed.

Unit Tests:

• I have added or updated unit tests for my change. (N/A: typo-only fix in a prompt string literal; the module's tests mock the templates and do not snapshot the prompt text.)
• All unit tests pass locally.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• New and existing unit tests pass locally with my changes.