[PE Dry Run]sort command does not work

[nus-se-bot]

sort student command does not work, even if condition is met (i.e. parameters already existed in the field, but 'unknown command is thrown')

current behaviour:

• sort NAME shows unknown command, when followed with the example in the User Guide

expected behaviour:

• persons are sorted according to NAME

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Reported by: @darieca Severity: Not Specified

_{[original: nus-cs2103-AY1819S1/pe-dry-run#330]}