DYN-7082 Update DSCore.List.IndexOf node help doc

[reddyashish]

Purpose

Update node help doc for List.IndexOf node.

Declarations

Check these if you believe they are true

• [ ] The codebase is in a better state after this PR
• [ ] Is documented according to the standards
• [ ] The level of testing this PR includes is appropriate
• [ ] User facing strings, if any, are extracted into *.resx files
• [ ] All tests pass using the self-service CI.
• [ ] Snapshot of UI changes, if any.
• [ ] Changes to the API follow Semantic Versioning and are documented in the API Changes document.
• [ ] This PR modifies some build requirements and the readme is updated
• [ ] This PR contains no files larger than 50 MB

Release Notes

Update DSCore.List.IndexOf node help doc

Reviewers

@QilongTang