Fix Typo - Update JSON_mode_example.ipynb

[yogeshkc]

Fix typo in "strucutre" word

Why are these changes needed?

Fix a typo in the documentation

Related issue number

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• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md 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.