Text splitter docs and colab update

[fm1320]

What does this PR do?

Fixes #<issue_number>

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• Was this discussed/agreed via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
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[liyin2015]

as the code is from the existing tutorial, no need to review it if everything runs and in sync with tutorial