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

Merge #6305

**Please ensure you have read the [contribution guide](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) 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.)
- [x] All unit tests pass locally.

### Checklist

- [x] I have read the [CONTRIBUTING.md](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) document.
- [x] I have performed a self-review of my own code.
- [x] New and existing unit tests pass locally with my changes.

COPYBARA_INTEGRATE_REVIEW=#6305 from anxkhn:docs/simulator-prompt-typo 34dc49f
PiperOrigin-RevId: 943534249

Author: anxkhn

src/google/adk/evaluation/simulation/per_turn_user_simulator_quality_prompts.py

@@ -146,7 +146,7 @@
 
 # Definition of Conversation History
 The Conversation History is the actual dialogue between the User Simulator and the Agent.
-The Conversation History may not be complete, but the exsisting dialogue should adhere to the Conversation Plan.
+The Conversation History may not be complete, but the existing dialogue should adhere to the Conversation Plan.
 The Conversation History may contain instances where the User Simulator troubleshoots an incorrect/inappropriate response from the Agent in order to enforce the Conversation Plan.
 The Conversation History is finished only when the User Simulator outputs `{{ stop_signal }}` in its response. If this token is missing, the conversation between the User Simulator and the Agent has not finished, and more turns can be generated.