fix: default should be T[] when multiple: true

[quantumkid]

Complementary pull request to oclif/parser#56.

[salesforce-cla]

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[codecov]

Codecov Report

Merging #70 into master will increase coverage by 0.46%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #70      +/-   ##
==========================================
+ Coverage   68.67%   69.13%   +0.46%     
==========================================
  Files           5        5              
  Lines         166      162       -4     
  Branches       35       35              
==========================================
- Hits          114      112       -2     
+ Misses         32       30       -2     
  Partials       20       20

Impacted Files	Coverage Δ
src/flags.ts	55% <ø> (ø)	⬆️
src/index.ts	77.77% <0%> (+0.5%)	⬆️
src/util.ts	44.44% <0%> (+5.55%)	⬆️

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