Update List.IndexOf node description

[QilongTang]

Please Note:

1. Before submitting the PR, please review How to Contribute to Dynamo
2. Dynamo Team will meet 1x a month to review PRs found on Github (Issues will be handled separately)
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Purpose

Per customer request, updating node description for List.IndexOf node. See UI change below (notice word hard was later refactored to be strict):

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

Reviewers

@DynamoDS/dynamo @Amoursol

FYIs

[QilongTang]

@Amoursol @aparajit-pratap Addressed the comments. If looks good, please approve.

[Amoursol]

LGTM @QilongTang - did you need me to officially commit my review?