docs: use idxs instead of vars for plotting in tutorials

[sathvikbhagavan]

This is to avoid this warning popping up in the docs:

┌ Warning: To maintain consistency with solution indexing, keyword argument vars will be removed in a future version. Please use keyword argument idxs instead.
│   caller = ip:0x0
└ @ Core :-1

[ai-maintainer]

AI-Maintainer Review for PR - docs: use idxs instead of vars for plotting in tutorials

Title and Description 👍

The Title and description are clear, concise and helpful

The title and description of the pull request are clear and concise. They effectively communicate the purpose of the changes, which is to replace the vars keyword argument with idxs in the plotting functions in tutorials to avoid a warning message and ensure future compatibility.

Scope of Changes 👍

The changes are narrowly focused

The changes in this pull request are narrowly focused on a specific issue. The author is not trying to resolve multiple issues simultaneously. The changes are targeted towards addressing the warning message related to the vars keyword argument and ensuring future compatibility by using the recommended idxs keyword argument instead.

Testing ⚠️

Testing details are missing

The description of the pull request does not mention how the author tested the changes. It is recommended for the author to include information about the testing approach in the pull request description. This could involve mentioning the specific steps taken to test the modified code, any test cases used, or any additional tools or frameworks utilized for testing.

Suggested Changes

No new functions, classes, or methods have been added, so no new docstrings are required. However, it would be beneficial to include details about how the changes were tested to ensure the quality and reliability of the changes.

Please add details about the testing process to the pull request description. This could include the specific steps taken to test the modified code, any test cases used, or any additional tools or frameworks utilized for testing.

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