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

Correct 'exsisting' to 'existing' in the Conversation History section of the with-persona evaluator prompt template. The sibling non-persona template already uses the correct spelling. This text is sent to the model at evaluation time.

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.