Update getting_started.md

[Siyeong-Lee]

L268: [wrong argumentation]: --work_dir --> --work-dir

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #3387 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3387      +/-   ##
==========================================
- Coverage   57.97%   57.94%   -0.04%     
==========================================
  Files         202      202              
  Lines       13491    13491              
  Branches     2282     2282              
==========================================
- Hits         7822     7817       -5     
- Misses       5310     5315       +5     
  Partials      359      359              

Flag	Coverage Δ
#unittests	57.94% <ø> (-0.04%)	⬇️

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Impacted Files	Coverage Δ
mmdet/models/roi_heads/test_mixins.py	48.38% <0.00%> (-5.38%)	⬇️

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