Sort command does not work

[nus-se-bot]

Describe the bug Sort function does not work, even though it is stated in the User Guide.

To Reproduce Steps to reproduce the behavior:

1. Type Sort
2. It will show command unknown

Expected behavior List of persons will be sorted by their names

Screenshots

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Reported by: @jamessspanggg Severity: High

_{[original: nus-cs2103-AY1819S2/pe-dry-run#54]}