Fix typo in websockets notebook

[davorrunje]

Why are these changes needed?

Typo in the websockets notebook is fixed.

Related issue number

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 ✅

Project coverage is 38.14%. Comparing base (e513a18) to head (20e0174).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2287   +/-   ##
=======================================
  Coverage   38.14%   38.14%           
=======================================
  Files          78       78           
  Lines        7859     7859           
  Branches     1680     1680           
=======================================
  Hits         2998     2998           
  Misses       4611     4611           
  Partials      250      250           

Flag	Coverage Δ
unittests	38.13% <ø> (ø)

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