Fix Langchain link in examples doc

[hodgesmr]

Why are these changes needed?

The link to the Notebook associated with "Task Solving with Langchain Provided Tools as Functions" was pointing to the incorrect url.

Related issue number

N/a

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.

[qingyun-wu]

LGTM. Thanks for the fix!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8d5f176) 27.79% compared to head (bc95465) 27.79%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #811   +/-   ##
=======================================
  Coverage   27.79%   27.79%           
=======================================
  Files          27       27           
  Lines        3497     3497           
  Branches      793      793           
=======================================
  Hits          972      972           
  Misses       2454     2454           
  Partials       71       71           

Flag	Coverage Δ
unittests	27.73% <ø> (ø)

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