Update agentchat_oai_assistant_retrieval.ipynb

[gagb]

Why are these changes needed?

change our -> your in retrieval nb for gpt assistant to reduce confusion

Related issue number

#731

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d22664f) 27.89% compared to head (ddf999c) 27.89%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #732   +/-   ##
=======================================
  Coverage   27.89%   27.89%           
=======================================
  Files          27       27           
  Lines        3466     3466           
  Branches      784      784           
=======================================
  Hits          967      967           
  Misses       2428     2428           
  Partials       71       71           

Flag	Coverage Δ
unittests	27.84% <ø> (ø)

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