Invalid printed index of IndexParser

[nus-pe-bot]

Indexing is not intuitive for users.

called delete_task 1 but error message shows "invalid index. task with index 2 does not exist."

This is especially confusing because your UG usually references the indexes starting from 1,2,3...
but your commands use the zero-based-indexing

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_{[original: kwekke/ped#6]}

[ghost]

Fixed in #211, documentation should reflect that we use zero-based indexing.

[Kyzure]

Issue solved now error is also 0-based index. Added in documentation that it is zero-based indexing as well.