docs: fix 'retreving' typo in FunctionNode.model_copy comment

[anxkhn]

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

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• N/A (small typo fix)

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

Problem:
An inline comment in FunctionNode.model_copy misspells "retrieving" as
"retreving":

# src/google/adk/workflow/_function_node.py
# Like Workflow or LLM use that name for event node_paths or retreving session events.

Solution:
Correct the single misspelled word retreving -> retrieving. This is a
comment-only change: no code, behavior, or public API is affected.

-    # Like Workflow or LLM use that name for event node_paths or retreving session events.
+    # Like Workflow or LLM use that name for event node_paths or retrieving session events.

Testing Plan

This is a small comment/typo fix with no runtime impact, so no unit tests were
added or changed. Verification was limited to spell-check and the repo's
pre-commit hooks on the touched file:

• codespell src/google/adk/workflow/_function_node.py -> passes (the tool that
  flags this misspelling reports the file clean after the fix).
• pre-commit run --files src/google/adk/workflow/_function_node.py -> all hooks
  pass (ruff, isort, pyink, addlicense, EOF/whitespace, compliance checks).

Unit Tests:

• I have added or updated unit tests for my change. (N/A - comment-only typo fix)
• All unit tests pass locally. (No tests affected; touched file is comment-only.)

Manual End-to-End (E2E) Tests:

N/A. The change only edits a source comment and cannot alter runtime behavior.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas. (N/A - this fixes an existing comment.)
• I have added tests that prove my fix is effective or that my feature works. (N/A - comment-only typo fix.)
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end. (N/A - no runtime behavior changes.)
• Any dependent changes have been merged and published in downstream modules. (N/A.)

Additional context

git grep retreving returns no other occurrences; this was the only instance of
the misspelling in the repository.