Fix update feature bugs

[AudreyFelicio]

No description provided.

[codecov-io]

Codecov Report

Merging #258 into master will increase coverage by 0.03%.
The diff coverage is 55.55%.

@@             Coverage Diff              @@
##             master     #258      +/-   ##
============================================
+ Coverage     60.50%   60.54%   +0.03%     
- Complexity      809      810       +1     
============================================
  Files           127      127              
  Lines          3178     3183       +5     
  Branches        420      422       +2     
============================================
+ Hits           1923     1927       +4     
- Misses         1119     1121       +2     
+ Partials        136      135       -1     

Impacted Files	Coverage Δ	Complexity Δ
...java/seedu/stock/logic/commands/UpdateCommand.java	76.85% <0.00%> (ø)	16.00 <0.00> (ø)
...rc/main/java/seedu/stock/model/stock/Quantity.java	92.85% <ø> (ø)	12.00 <0.00> (ø)
...in/java/seedu/stock/model/stock/QuantityAdder.java	45.45% <0.00%> (-7.18%)	4.00 <0.00> (ø)
.../seedu/stock/logic/parser/UpdateCommandParser.java	83.33% <100.00%> (+0.64%)	13.00 <0.00> (ø)
...du/stock/logic/parser/SuggestionCommandParser.java	83.44% <0.00%> (+0.66%)	72.00% <0.00%> (+1.00%)

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[kaichaoang]

LGTM!