[docs] Fix code examples and minor issues in DataStream documentation

[Vishal-Kamlapure]

What is the purpose of the change

This PR fixes several issues in the DataStream documentation.

Brief change log

• Fixed incorrect code examples (MapFunction usage, test harness types)
• Corrected typos and grammar issues
• Fixed broken examples (missing parentheses, incorrect variables)
• Replaced deprecated generateSequence with fromSequence based on Flink stable documentation

Verifying this change

This change is a documentation and example fix. No functional changes were introduced.

[flinkbot]

CI report:

• 0f5453f Azure: FAILURE

Bot commands

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• @flinkbot run azure re-run the last Azure build

[reswqa]

Thank for the contribution. Can you also apply the fix(code part) to Chinese doc.

[Vishal-Kamlapure]

Thanks for the suggestion, @reswqa ! I’ve applied the same code fixes to the Chinese documentation as well.