Delete 1 q/20 not working

[nus-pe-script]

Describe the bug
Invalid Command delete 1 q/20

To Reproduce
Steps to reproduce the behavior:

1. Enter 'delete 1 q/10
   
   '

Expected behavior
decreases the quantity of the 1st item by 20

Screenshots
If applicable, add screenshots to help explain your problem.

Additional context
Add any other context about the problem here.

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Reported by: @kennethcsj
Severity: High

_{[original: nusCS2113-AY1819S1/pe-1#26]}

[gaoqikai]

fixed, changed user guide error